Intermediate `block-stream`, with 'node-name' for 'base' parameter
==================================================================

Goal: 

Given, the chain: 

    [A] <-- [B] <-- [C] <-- [D]
            
Merge contents of images [B] into [C], where [C] ends up referring to
[A] as its backing image, and the final image chain will end up being:

    [A] <-- [C] <-- [D]

#----------------------------------------------------------------------
(1) Launch QEMU as following:

$ ~/build/qemu-upstream/x86_64-softmmu/qemu-system-x86_64 -M q35 \
    -display none -nodefconfig -nodefaults -m 512 \
    -blockdev node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \
    -device virtio-blk,drive=node-A,id=virtio0 \
    -S -monitor stdio -qmp unix:./qmp-sock,server,nowait
#----------------------------------------------------------------------

(2) Create three overlays:

(QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2
(QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2
(QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2
#----------------------------------------------------------------------

(3) Check the backing file depth, it is indeed: A <- B <- C <- D:

    $ qemu-img info --backing-chain d.qcow2  | grep "backing file:"
    backing file: c.qcow2
    backing file: b.qcow2
    backing file: ./a.qcow2
#----------------------------------------------------------------------

(4) Perform intermediate streaming:

(QEMU) block-stream device=node-C base-node=node-A job-id=job0                                                                                                                     
{
    "execute": "block-stream", 
    "arguments": {
        "device": "node-D", 
        "job-id": "job0", 
        "base-node": "node-A"
    }
}
{
    "return": {}
}
(QEMU) 
{u'timestamp': {u'seconds': 1496325065, u'microseconds': 808768}, u'data': {u'device': u'job0', u'type': u'stream', u'speed': 0, u'len': 41126400, u'offset': 41126400}, u'event': u'BLOCK_JOB_COMPLETED'}
#----------------------------------------------------------------------

(5) Check the backing file depth, it is now: A <- C <- D:

$ qemu-img info --backing-chain d.qcow2  | grep "backing file:"
backing file: c.qcow2
backing file: ./a.qcow2
#----------------------------------------------------------------------