docs: apps: Update references to virt-p2v and virt-v2v.

docs: apps: Update references to virt-p2v and virt-v2v.

These tools have been rewritten upstream, so you don't need to link to
the old tools, link to the new ones and mention they are part of
libguestfs.

Also remove the link to "Poor man's P2V".  There's no real reason to
use that technique any longer since the rewritten tools are simple,
fast and highly capable.