[libvirt] Tunnelled migration

At long last, now that the stream stuff is committed to the tree, I can repost the tunnelled migration patches. Note that these patches will conflict in some places with danpb's previously posted "PEER2PEER" patches, but it should be easy to resolve the differences. V2: Rework the monitor patch to be an API Use the new monitor API Remove silly use of qemuEscapeMonitorArgs()