The event loop implementation is used by more than just the
daemon, so move it into the shared area.

* daemon/event.c, src/util/event_poll.c: Renamed
* daemon/event.h, src/util/event_poll.h: Renamed
* tools/Makefile.am, tools/console.c, tools/virsh.c: Update
  to use new virEventPoll APIs
* daemon/mdns.c, daemon/mdns.c, daemon/Makefile.am: Update
  to use new virEventPoll APIs