If the image is read-only then it's not possible to copy read data into
it.  Therefore copy-on-read is automatically disabled for read-only
images.

Up until now this behavior was silent, add a warning so the user knows
why copy-on-read is not working.

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