[Libguestfs] Refactor virt-v2v to be more like a 'big script'

These patches combine HVSource and HVTarget into a single Converter. This should make it more obvious where to hack without losing any practical flexibility. GuestOS remains separate. GuestOS is now a misnomer, because it's really only a Linux distro abstraction. It will be useless for Windows, for example. Functions which you'd expect to be different on a non-RH distro should live in GuestOS. By this measure it's currently too fat. I'll gradually move non-distro specific things out of it as I touch them. Matt