QMP: Asynchronous events infrastructure

QMP: Asynchronous events infrastructure

Asynchronous events are generated with a call to
monitor_protocol_event().

This function builds the right data-type and emit the event
right away. The emitted data is always a JSON object and its
format is as follows:

{ "event": json-string,
  "timestamp": { "seconds": json-number, "microseconds": json-number },
  "data": json-value }

This design is based on ideas by Amit Shah <amit.shah@redhat.com>.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>