[libvirt] Guestfwd support (round 3)

This is substantially the same as the last set. I don't think I changed patches 1 and 2 at all. Patch 3 is updated to use a virBuffer as Rich Jones recommended. Patch 4 is updated slightly for the above change, and there are a couple of other trivial tidy ups in there. Patch 5 is new. QEMU's guestfwd only supports IPv4 addresses, so we check for that. Rich Jones also noted that guestfwd is specific to QEMU. If there's a good chance this same syntax might do something simililar in a different driver, I agree that changing the name would be a good idea. However, I'm not familiar enough with other hypervisors to be able to make a call on this. If nothing else does anything similar, I'd keep the QEMU name for clarity. Matt