Both 030 and 041 use create_image().  Move it to iotests.py.

Also drop ImageStreamingTestCase since the class now has no methods.
Suggested-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>