[Libguestfs] support remotely mounting disk images in p2v

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.