QMP: Fix asynchronous events delivery

Commit f039a563f200beee80cc10fd70b21ea396979dab introduces
a regression as monitor_protocol_event() will return in
the first user Monitor it finds in the QLIST_FOREACH()
loop.

The right thing to do is to only delivery an asynchronous
event if the 'mon' is a QMP Monitor.

The aforementioned commit was an early version, if it was
applied to stable (it should) this one has to be applied
there too.

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