virt: make domain_descriptor.py API pep8-friendly

Only changes in naming.

Change-Id: I6aadabb0152966dae7da885b6933c440d6496f6d
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33672
Reviewed-by: Dan Kenigsberg <danken@redhat.com>