monitor: add an implemention of qapi event emit method

monitor: add an implemention of qapi event emit method

The monitor is now hooked on the new event mechanism, so that later
patches can convert event callers one by one. Most code are copied from
old monitor_protocol_* functions with some modification.

Note that two build time warnings will be raised after this patch. One is
caused by no caller of monitor_qapi_event_throttle(), the other one is
caused by QAPI_EVENT_MAX = 0. They will be fixed automatically after
full event conversion later.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>