The Xen & VBox drivers deal with callbacks & dispatching of
events directly. All the other drivers use a timer to dispatch
events from a clean stack state, rather than deep inside the
drivers. Convert Xen & VBox over to virDomainEventState so
that they match behaviour of other drivers

* src/conf/domain_event.c: Return count of remaining
  callbacks when unregistering event callback
* src/vbox/vbox_tmpl.c, src/xen/xen_driver.c,
  src/xen/xen_driver.h: Convert to virDomainEventState