[libvirt] [PATCH] Account for defined networks when generating bridge names

This is the second part of the fix for rhbz 479622. If we are generating a bridge name for a virtual network, don't collide with any bridge name in a defined network. This patch also generates a bridge name at network define time, if none was passed in the xml. The downside to all this is that it won't fix things for existing victims of the bug: if they have 2 networks with the same bridge device in the xml, we can't intelligently remedy the situation. This patch just helps prevent future users getting into that predicament. Thanks, Cole