Event power integration

It seems to be working quite nicely. CFME correctly pushes the requests into the listener and the pickup works. There is, however,this problem:
* ``vm_power_off_req`` event is not invoked for some reason on power off. I didn't study it thoroughly, maybe it has some deeper meaning.

I've changed the register_event fixture to be a yield_fixture to make it possible to make individual setups and teardowns for each function using it.

I also changed the explicit 1m wait for wait_for. Now it queries the listener more, but it's likely to finish sooner (if all events come on time). The VM power control tests however use sleeps, so I will try to get rid of them,