qemu-iotests: Test streaming with missing job ID

This adds a small test for the image streaming error path for failing
block_job_create(), which would have found the null pointer dereference
in commit a170a91f.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
