The iotests.compare_images() function returns True if two image files
have the identical data.  Previously this was implemented by converting
images to raw and then comparing their contents using Python.  Since
"qemu-img compare" is now available and is more efficient, switch to it.

This function will be reused by the 'drive-backup' test case.
Suggested-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>