qemu-iotests: make compare_images() common

qemu-iotests: make compare_images() common

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: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>