[HMP] Before `block-commit`
---------------------------
(qemu) info block
virtio0 (#block2287): d.qcow2 (qcow2)
    Cache mode:       writeback
    Backing file:     c.qcow2 (chain depth: 3)


[QMP] Perform active block commit on the 'qmp-shell'
---------------------------------------------------

Given the chain [A] <- [B] <- [C] <- [D]

We're committing the contents of images [B], [C], [D] into [A]:

(QEMU) block-commit device=virtio0 base=disk1.qcow2 top=d.qcow2 job-id=job0                                                                                                                   
{
    "execute": "block-commit", 
    "arguments": {
        "device": "virtio0", 
        "job-id": "job0", 
        "top": "d.qcow2", 
        "base": "disk1.qcow2"
    }
}
{
    "return": {}
}
(QEMU) 
{u'timestamp': {u'seconds': 1495720462, u'microseconds': 252778}, u'data': {u'device': u'job0', u'type': u'commit', u'speed': 0, u'len': 786432, u'offset': 786432}, u'event': u'BLOCK_JOB_READY'}
(QEMU)


[HMP] After `block-commit`
--------------------------

*Observation*: Notice how it says 'd' *itself* is the backing file of
'd', which is meaningless.

(qemu) info block
virtio0 (#block2323): d.qcow2 (mirror_top)
    Cache mode:       writeback
    Backing file:     d.qcow2 (chain depth: 4)
(qemu) 




[QMP] Version
-------------

(QEMU) query-version
{
    "execute": "query-version", 
    "arguments": {}
}
{
    "return": {
        "qemu": {
            "micro": 50, 
            "major": 2, 
            "minor": 9
        }, 
        "package": " (v2.9.0-557-g0e17187-dirty)"
    }