Goal: Create a (live) QCOW2 overlay, and if the overlay size exceeds a certain
      threshold, then

(1) Launch QEMU's '-blockdev' with "file.node-name" (ensure either to
    set "share-rw=on" in the '-device' command-line; or just completely
    remove -- for testing only -- '-device' to be able to write to it.)

      $> qemu-system-x86_64 \
      -display none                  \
      -no-user-config                \
      -nodefaults                    \
      -monitor stdio                 \
      -cpu IvyBridge-IBRS            \
      -smp 1,maxcpus=2               \
      -machine q35,accel=kvm,usb=off \
      -m 2048                        \
      -blockdev node-name=node-Base,driver=qcow2,file.driver=file,file.filename=./base.qcow2,file.node-name=node-Base-file \
      -qmp unix:/tmp/qmp-sock,server,nowait

(2) Set the 'write-threshold' to 40 MB (41943040 bytes):

    $> ./qmp-shell -v -p /tmp/qmp-sock	
    (QEMU) block-set-write-threshold node-name=node-Foo-file write-threshold=41943040
    {
        "execute": "block-set-write-threshold",
        "arguments": {
            "node-name": "node-Foo-file",
            "write-threshold": 41943040
        }                                                                                                                                                                                     
    }                                                                                                                                                                                         

(3) If the 'write-threshold' exceeds 40MB, then you should an event
    BLOCK_WRITE_THRESHOLD.  Let's test it.

    From the HMP (not QMP) monitor, run the 'qemu-io' command to write
    40MB of data

      (qemu) qemu-io node-Foo "write -P 01 0 40943040"
      wrote 40943040/40943040 bytes at offset 0
      39.046 MiB, 1 ops; 0:00:01.11 (35.175 MiB/sec and 0.9009 ops/sec)

(4) Re-run the above 'qemu-io' HMP command a couple of times to exceed
    the threshold of 40MB.  Then, indeed, I get the
    BLOCK_WRITE_THRESHOLD event as expected (intentionally line-wrapping
    the event):

      {u'timestamp': {u'seconds': 1557408333, u'microseconds': 826608},
      u'data': {u'node-name': u'node-Foo-file', u'write-threshold':
      41943040, u'amount-exceeded': 7126464}, u'event':
      u'BLOCK_WRITE_THRESHOLD'}

(5) And then based on that event, do a `block-commit` of the overlay
    into the backing file.