Collision Possible When Assigning Port For Libvirt

[ndonegan]

At the moment, there's a rare condition when the random port chosen by OZ for libvirt can conflict with already used ports. The chances of it happening are low, but it does happen. See https://github.com/clalancette/oz/blob/master/oz/Guest.py#L229

Suggested fix would be to use socket to bind to port 0 first, which will choose a random free port which can be used by listen_port. There's still the slight chance of a race condition where something listens between the bind and libvirt starting up, but the odds are a lot less. At the very least, it's not going to conflict with long running processes listening on tcp ports in that range.

[clalancette]

I've applied your patch, which hopefully should improve things. I'm actually going to leave this open to investigate having libvirt open the port for us, which would close the remaining race.