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

[Matthew Booth]

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