[Libguestfs] [PATCH virt-v2v 0/2] v2v: Split plugin functions and enable multi-conn

This patch is independent of https://listman.redhat.com/archives/libguestfs/2021-August/msg00009.html but probably needs to be applied on top of it (in any case I've only tested it on top). This doesn't make things any faster because qemu-img convert cannot use multi-conn (hence why the last patch is separate). But it doesn't seem to have any performance impact, so we may as well go with it. The patch isn't quite final because it breaks the test suite. The test suite relies on a single process running the mock ovirtsdk4 replacement API, because it creates a background thread that runs the phony web service (immitating imageio). I'm going to need to think about how to do it now that there are several different scripts involved. However I did test it against real RHV 4.4 and it seems to be OK. Rich.