qemu-iotests/041: Fix leaked scratch images

qemu-iotests 041 left quorum_snapshot.img and target.img behind in the
scratch directory. Make sure to clean up after completing the tests.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
