Tighten up scope after the previous patch avoided using
internals.  This will also make it easier to change
internal implementation without having to chase down quite
as many impacted callers or worrying about two files getting
implementations out of sync.

* src/conf/object_event_private.h
(virObjectEventCallbackListAddID, virObjectEventQueueClear)
(virObjectEventStateLock, virObjectEventStateUnlock)
(virObjectEventTimer): Drop prototype.
(_virObjectEventCallbackList, _virObjectEventState)
(_virObjectEventCallback): Move...
* src/conf/object_event.c: ...here.
(virObjectEventCallbackListAddID, virObjectEventQueueClear)
(virObjectEventStateLock, virObjectEventStateUnlock)
(virObjectEventTimer): Mark private.
Signed-off-by: NEric Blake <eblake@redhat.com>