iotests: Make 184 image-less

184 does not need an image, so don't use one.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171123020832.8165-16-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>