Taken from virt-manager code. Move it here because it is strictly
an XML operation, and it will be easier to unit test
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Move the opencoded impl out of virt-manager details.py and into
virtinst, since this is entirely about XML comparison. Add tests for
it
Signed-off-by: Cole Robinson <crobinso@redhat.com>