vm: cleanup post-libvirt merge code

after the libvirt.py merge a couple of methods remained
dangling and with out-to-date comment.

This tiny patch drop unused code, and improve the documentation
of the still useful code with the current state and the feature
gap still open with libvirt.

Change-Id: I29e8a888d124fccd878a2ecac30734e58b355678
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24744
Reviewed-by: Dan Kenigsberg <danken@redhat.com>