Disks without media make no sense.  For SCSI, a Linux guest kernel
complains during boot.  I didn't try other combinations.

scsi-generic doesn't need the additional check, because it already
requires bdrv_is_sg(), which fails without media.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>