virt: Fix broken restore() functionality

* The backup/restore logic inherrited by xml_utils.XMLTreeFile
  is backwards from the perspective of a caller.
* xml_utils.XMLTreeFile must re-parse the tree after restoring
  from the backup.
* Provide restore() convenience method so test code doesn't
  need to access xmltreefile attribute directly.

Signed-off-by: Chris Evich <cevich@redhat.com>