Add APIs to allow management of callbacks purely with virDomainEventState

Add APIs to allow management of callbacks purely with virDomainEventState

While virDomainEventState has APIs for managing removal of callbacks,
while locked, adding callbacks in the first place requires direct
access to the virDomainEventCallbackList structure. This is not
threadsafe since it is bypassing the virDomainEventState locks

* src/conf/domain_event.c, src/conf/domain_event.h,
  src/libvirt_private.syms: Add APIs for managing callbacks
  via virDomainEventState.