Filtering monitor events by name requires tracking the name for
the duration of the filtering.  In order to free the name, I
found it easiest to just piggyback on the user's freecb function,
which gets called when the event is deregistered.

For events without a name filter, we have the design of multiple
client registrations sharing a common server registration, because
the server side uses the same callback function and we reject
duplicate use of the same function.  But with events in the mix,
we want to be able to allow the same function pointer to be used
with more than one event name.  The solution is to tweak the
duplicate detection code to only act when there is no additional
filtering; if name filtering is in use, there is exactly one
client registration per server registration.  Yes, this means
that there is no longer a bound on the number of server
registrations possible, so a malicious client could repeatedly
register for the same name event to exhaust server memory.  On
the other hand, we already restricted monitor events to require
write access (compared to normal events only needing read access),
and separated it into the intentionally unsupported
libvirt-qemu.so, with documentation that using this function is
for debug purposes only; so it is not a security risk worth
worrying about a client trying to abuse multiple registrations.

* src/conf/domain_event.c (virDomainQemuMonitorEventData): New
struct.
(virDomainQemuMonitorEventFilter)
(virDomainQemuMonitorEventCleanup): New functions.
(virDomainQemuMonitorEventDispatchFunc)
(virDomainQemuMonitorEventStateRegisterID): Use new struct.
* src/conf/object_event.c (virObjectEventCallbackListCount)
(virObjectEventCallbackListAddID)
(virObjectEventCallbackListRemoveID)
(virObjectEventCallbackListMarkDeleteID): Drop duplicate detection
when filtering is in effect.
Signed-off-by: NEric Blake <eblake@redhat.com>