virttest.env_process: Fix backup_image behavior

Using the test runner defaults, instead of backing up
an image at the end of an unattended install, we'll
end up restoring a (potentially bad) backup instead.

So if backup_image is set to 'yes', ignore restore_image.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>