[27-June-2018: Refer to:
https://kashyapc.fedorapeople.org/virt/qemu/NBD-export-of-a-PIT-snapshot-of-a-disk-with-QEMU-2.8.txt]

Use-case:

    Provide an NBD export that serves you a (read-only?) point-in-time
    (PIT) snapshot of a disk.  [Also sometimes referred to as "Image
    Fleecing"]

Procedure:

-----------------------------------------------------------------------
(1) Create the target image ('drive1', with 'drive0' as its backing
    file) using 'blockdev-add'

$ qemu-img create -f qcow2 target.qcow2 1G

$ ./qmp-shell -v -p /export/qemu-backup-tests/qmp-sock
(QEMU) blockdev-add options={"id":"drive-ide1-0-0","driver":"qcow2","file":{"driver":"file","filename":"/export/target.qcow2"},"backing":"drive-ide0-0-0"}


(2) Start the built-in NBD server over UNIX socket 'nbd-server-start'

    (QEMU) nbd-server-start addr={"type":"unix","data":{"path":"./nbd-sock"}}

(3) Take the backup of device 'drive0' into 'drive1' using
    'blockdev-backup'

(QEMU) blockdev-backup device=drive-ide0-0-0 target=drive-ide1-0-0 sync=none

(4) Then, export 'drive1' over NBD with 'nbd-server-add'

(QEMU) nbd-server-add device=drive-ide1-0-0

(5) Discard the NBD export at anytime you wish (with 'nbd-server-stop'):

(QEMU) nbd-server-stop
-----------------------------------------------------------------------

    * * *

=== Same as above, but with QMP JSON output ===

(1) Add block device with a backing file

-----------------
$ qemu-img create -f qcow2 target.qcow2 1G
-----------------
(QEMU) blockdev-add options={"id":"drive-ide1-0-0","driver":"qcow2","file":{"driver":"file","filename":"/export/target.qcow2"},"backing":"drive-ide0-0-0"}
{
    "execute": "blockdev-add", 
    "arguments": {
        "options": {
            "backing": "drive-ide0-0-0", 
            "driver": "qcow2", 
            "id": "drive-ide1-0-0", 
            "file": {
                "driver": "file", 
                "filename": "/export/target.qcow2"
            }
        }
    }
}
{
    "return": {}
}
(QEMU) 
-----------------
(qemu) info block
drive-ide0-0-0 (#block123): ./cirros-0.3.3.qcow2 (qcow2)
    Cache mode:       writeback

drive-ide1-0-0 (#block360): /export/target.qcow2 (qcow2)
    Removable device: not locked, tray closed
    Cache mode:       writeback
    Backing file:     ./cirros-0.3.3.qcow2 (chain depth: 1)
(qemu) 
-----------------

(2) NBD server start

(QEMU) nbd-server-start addr={"type":"unix","data":{"path":"./nbd-sock"}}
{
    "execute": "nbd-server-start", 
    "arguments": {
        "addr": {
            "data": {
                "path": "./nbd-sock"
            }, 
            "type": "unix"
        }
    }
}
{
    "return": {}
}
(QEMU) 


(3) Take the backup of the backing file to the target drive:

(QEMU) blockdev-backup device=drive-ide0-0-0 target=drive-ide1-0-0 sync=none
{
    "execute": "blockdev-backup", 
    "arguments": {
        "device": "drive-ide0-0-0", 
        "target": "drive-ide1-0-0", 
        "sync": "none"
    }
}
{
    "error": {
        "class": "GenericError", 
        "desc": "Node 'drive-ide0-0-0' is busy: node is used as backing hd of '#block360'"
    }
}
(QEMU)



[PATCH v24 01/12] unblock backup operations in backing file -- 
https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg06096.html
-----------------------------------------------------------------------
(QEMU) blockdev-backup device=drive-ide0-0-0 target=drive-ide1-0-0 sync=none
{
    "execute": "blockdev-backup",
    "arguments": {
        "device": "drive-ide0-0-0",
        "target": "drive-ide1-0-0",
        "sync": "none"
    }
}
{
    "return": {}
}
-----------------------------------------------------------------------
(QEMU) query-block-jobs
{
    "execute": "query-block-jobs", 
    "arguments": {}
}
{
    "return": [
        {
            "busy": false, 
            "type": "backup", 
            "len": 41126400, 
            "paused": false, 
            "ready": false, 
            "io-status": "ok", 
            "offset": 0, 
            "device": "drive-ide0-0-0", 
            "speed": 0
        }
    ]
}
(QEMU)
-----------------------------------------------------------------------
(QEMU) nbd-server-start addr={"type":"unix","data":{"path":"./nbd-sock"}}
{
    "execute": "nbd-server-start", 
    "arguments": {
        "addr": {
            "data": {
                "path": "./nbd-sock"
            }, 
            "type": "unix"
        }
    }
}
{
    "return": {}
}
-----------------------------------------------------------------------
(QEMU) nbd-server-add device=drive-ide1-0-0
{
    "execute": "nbd-server-add", 
    "arguments": {
        "device": "drive-ide1-0-0"
    }
}
{
    "return": {}
}
(QEMU) 
-----------------------------------------------------------------------