[Libguestfs] support remotely mounting disk images in p2v

[Mo Morsi]

Figure I'd share the latest revision to p2v I had regarding $subject.

The patch now applies against HEAD and compiles fine. I was able to update
the package on the client and run virt-p2v, though have yet to do a full
end-to-end verification

Most likely a little more work is needed to tidy up some edge cases and fully
flush things out, but the majority of the work should be in place.

On the client side the disk is exported using qemu-nbd and an new ssh forwarding
channel is opened up infront of it. The server simply instructs libguestfs to connect
to nbd://localhost:port/ and the rest is taken care of.