The virtual box driver was directly accesing the domain events
structs instead of using the APIs provided. To prevent this kind
of abuse, make the struct definitions private, forcing use of the
internal APIs. This requires adding one extra internal API.

* src/conf/domain_event.h, src/conf/domain_event.c: Move
  virDomainEventCallback and virDomainEvent structs into
  the source file instead of header
* src/vbox/vbox_tmpl.c: Use official APIs for dispatching domain
  events instead of accessing structs directly.