Create qemu monitor events as a distinct class to normal domain
events, because they will be filtered differently.  For ease of
review, the logic for filtering by event name is saved for a later
patch.

* src/conf/domain_event.c (virDomainQemuMonitorEventClass): New
class.
(virDomainEventsOnceInit): Register it.
(virDomainQemuMonitorEventDispose, virDomainQemuMonitorEventNew)
(virDomainQemuMonitorEventDispatchFunc)
(virDomainQemuMonitorEventStateRegisterID): New functions.
* src/conf/domain_event.h (virDomainQemuMonitorEventNew)
(virDomainQemuMonitorEventStateRegisterID): New prototypes.
* src/libvirt_private.syms (conf/domain_conf.h): Export them.