Let's report specific errors so that management tools and users can
identify the problem.

Two new qerrors are needed:
 * QERR_DEVICE_HAS_NO_MEDIUM for ENOMEDIUM
 * QERR_DEVICE_IS_READ_ONLY for EACCES
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>