Add event notification for guest balloon changes

After setting a balloon target value, applications have to
continually poll 'query-balloon' to determine whether the
guest has reacted to this request. The virtio-balloon backend
knows exactly when the guest has reacted though, and thus it
is possible to emit a JSON event to tell the mgmt application
whenever the guest balloon changes.

This introduces a new 'qemu_balloon_changed()' API which is
to be called by balloon driver backends, whenever they have
a change in balloon value. This takes the 'actual' balloon
value, as would be found in the BalloonInfo struct.

The qemu_balloon_change API emits a JSON monitor event which
looks like:

  {"timestamp": {"seconds": 1337162462, "microseconds": 814521},
   "event": "BALLOON_CHANGE", "data": {"actual": 944766976}}

* balloon.c, balloon.h: Introduce qemu_balloon_changed() for
  emitting balloon change events on the monitor
* hw/virtio-balloon.c: Invoke qemu_balloon_changed() whenever
  the guest changes the balloon actual value
* monitor.c, monitor.h: Define QEVENT_BALLOON_CHANGE

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Acked-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

QMP/qmp-events.txt

@@ -335,3 +335,21 @@ Example:
                "len": 10737418240, "offset": 134217728,
                "speed": 0 },
      "timestamp": { "seconds": 1267061043, "microseconds": 959568 } }
+
+
+BALLOON_CHANGE
+----------
+
+Emitted when the guest changes the actual BALLOON level. This
+value is equivalent to the 'actual' field return by the
+'query-balloon' command
+
+Data:
+
+- "actual": actual level of the guest memory balloon in bytes (json-number)
+
+Example:
+
+{ "event": "BALLOON_CHANGE",
+    "data": { "actual": 944766976 },
+    "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }