The virDomainEventCallbackList and virDomainEventQueue APIs are
now solely helpers used internally by virDomainEventState APIs.
Remove their decls from domain_event.h since no driver code should
need to use them any more.

* src/conf/domain_event.c: Make virDomainEventCallbackList and
  virDomainEventQueue APIs static & remove some unused APIs
* src/conf/domain_event.h, src/libvirt_private.syms: Remove
  virDomainEventCallbackList and virDomainEventQueue APIs