v2v: Rename -o (target) tests.

No functional change.
v2v: Add new -i disk to allow direct import of disk images with no metadata.

This is simpler to use and more convenient than -i libvirtxml, because
users don't need to bother with writing libvirt XML.  However it is
less expressive because many source capabilities such as memory and
number of vCPUs cannot be specified this way.