snapshot/tests: Try loadvm twice

It's legal to loadvm twice, modify the existing save/loadvm test
to do it twice.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170825141940.20740-3-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>