Purpose: Test NBD-based live migration (with
'paused-before-switchover'[*]) using raw JSON QMP commands.

    https://lists.gnu.org/archive/html/qemu-devel/2017-10/msg04895.html
    [PATCH v4 0/7] migration: pause-before-switchover

Tested with: 

    {"version": {"qemu": {"micro": 90, "minor": 10, "major": 2}, "package": "
    (v2.11.0-rc0-47-g55ed8d6)"}, "capabilities": []}}

(1) [SOURCE] and [DEST]: Launch source and destination QEMU on two
    different shells, run the `qmp_capabilities` command on source and
    destination.
-----------------------------------------------------------------------
[SOURCE]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$> ~/build/qemu-build/x86_64-softmmu/qemu-system-x86_64 -display none \
    -nodefconfig -nodefaults -m 512 -device virtio-scsi-pci,id=scsi \
    -device virtio-serial-pci \
    -drive file=./src-disk.qcow2,format=qcow2,if=virtio \
    -serial unix:/tmp/monitor,server,nowait -qmp stdio
{ "execute": "qmp_capabilities" }
{"return": {}}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[DEST]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$> qemu-img info src-disk.qcow2 
image: src-disk.qcow2
file format: qcow2
virtual size: 39M (41126400 bytes)
disk size: 14M
cluster_size: 65536
Format specific information:
    compat: 0.10
    refcount bits: 16
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$> qemu-img create -f qcow2 -o preallocation=falloc dst-disk.qcow2 40M
Formatting 'dst-disk.qcow2', fmt=qcow2 size=41943040 encryption=off cluster_size=65536 preallocation=falloc lazy_refcounts=off refcount_bits=16
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$> qemu-img info dst-disk.qcow2 
image: dst-disk.qcow2
file format: qcow2
virtual size: 40M (41943040 bytes)
disk size: 40M
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Note: we're not starting the destination with '-S'; just "-incoming"
# seems to suffice
$> ~/build/qemu-build/x86_64-softmmu/qemu-system-x86_64 -display none \
    -nodefconfig -nodefaults -m 512 -device virtio-scsi-pci,id=scsi \
    -device virtio-serial-pci \
    -drive file=./dst-disk.qcow2,format=qcow2,if=virtio \
    -serial unix:/tmp/monitor,server,nowait \
    -incoming tcp:localhost:6666 -qmp stdio
{ "execute": "qmp_capabilities" }
{"return": {}}
-----------------------------------------------------------------------


(2) [SOURCE] Set the "pause-before-switchover" capability.
-----------------------------------------------------------------------
{"execute":"migrate-set-capabilities","arguments":{"capabilities":[{"capability":"pause-before-switchover","state":true}]}}
{"return": {}}
-----------------------------------------------------------------------
{"execute":"query-migrate-capabilities"}
{"return": [{"state": false, "capability": "xbzrle"}, {"state": false, "capability": "rdma-pin-all"}, {"state": false, "capability": "auto-converge"}, {"state": false, "capability": "zero-blocks"}, {"state": false, "capability": "compress"}, {"state": false, "capability": "events"}, {"state": false, "capability": "postcopy-ram"}, {"state": false, "capability": "x-colo"}, {"state": false, "capability": "release-ram"}, {"state": false, "capability": "block"}, {"state": false, "capability": "return-path"}, {"state": true, "capability": "pause-before-switchover"}, {"state": false, "capability": "x-multifd"}]}
-----------------------------------------------------------------------


(3) [DEST] Start the NBD server
-----------------------------------------------------------------------
{ "execute": "nbd-server-start", "arguments": { "addr": { "type": "inet","data": { "host": "localhost", "port": "3333" } } } }
{"return": {}}
{ "execute": "nbd-server-add", "arguments": { "device": "virtio0","writable": true } }
{"return": {}}
-----------------------------------------------------------------------


(4) [SOURCE] Execute the `drive-mirror` command
-----------------------------------------------------------------------
{ "execute": "drive-mirror", "arguments": { "device": "virtio0", "target": "nbd:localhost:3333:exportname=virtio0", "sync": "full","format": "raw", "mode": "existing" } }
{"return": {}}
{"timestamp": {"seconds": 1510677793, "microseconds": 645286}, "event": "BLOCK_JOB_READY", "data": {"device": "virtio0", "len": 41126400, "offset": 41126400, "speed": 0, "type": "mirror"}}
-----------------------------------------------------------------------


(5) [SOURCE] Issue the `migrate` command (and query the migration status):
-----------------------------------------------------------------------
{"execute":"migrate","arguments":{"blk":false,"inc":false,"uri":"tcp:localhost:6666"}}
{"return": {}}
{"timestamp": {"seconds": 1510744913, "microseconds": 630373}, "event": "STOP"}

{"execute":"query-migrate"}                                                                                                                                                              
{"return": {"expected-downtime": 300, "status": "pre-switchover", "setup-time": 2, "total-time": 110111, "ram": {"total": 537403392, "postcopy-requests": 0, "dirty-sync-count": 2, "page-size": 4096, "remaining": 4141056, "mbps": 268.56432, "transferred": 59759367, "duplicate": 115920, "dirty-pages-rate": 72829, "skipped": 0, "normal-bytes": 58601472, "normal": 14307}}}
-----------------------------------------------------------------------


(6) [SOURCE] Issue the `block-job-cancel` command
-----------------------------------------------------------------------
{"execute":"block-job-cancel","arguments":{"device":"virtio0"} }
{"return": {}}
{"timestamp": {"seconds": 1510678024, "microseconds": 526240}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "virtio0", "len": 41126400, "offset": 41126400, "speed": 0, "type": "mirror"}}
-----------------------------------------------------------------------


(7) [DEST] Stop the NBD server
-----------------------------------------------------------------------
{"execute":"nbd-server-stop"}
{"return": {}}
-----------------------------------------------------------------------


(8) [Optional][*] [DEST] Resume the guest vCPUs
-----------------------------------------------------------------------
{"execute":"cont"}
{"return": {}}
-----------------------------------------------------------------------


(9) [SOURCE] Gracefully terminate the QEMU process (repeat the same on
    destination.)
-----------------------------------------------------------------------
{"execute":"quit"}
{"return": {}}
{"timestamp": {"seconds": 1510678728, "microseconds": 574318}, "event": "SHUTDOWN", "data": {"guest": false}}
-----------------------------------------------------------------------

    * * *

[*] NOTE: If you started destination QEMU with "-S" (do not start CPU at
start up), then you have to issue the QMP 'cont' command on the
Detination.  We haven't in our case.  And Dave Gilbert says: when using
"-incoming" without "-S" on the destination, the migration code will
restart the CPUs automatically.