Avoid polling on FDs with no events enabled

Avoid polling on FDs with no events enabled

If a file descriptor with events=0 was added to the libvirtd
event loop, it would still be added to the poll() fds' array.
While it wouldn't see any POLLIN/OUT events, it'd still get
triggered for HANGUP/ERROR events which was not in compliance
with the libvirt events API contract.

* qemud/event.c: Don't poll on FDs with events=0
* tests/eventtest.c: Add test case to validate fix to event.c