block: Assume raw for drives without media

qemu -cdrom /dev/cdrom with an empty CD-ROM drive doesn't work any more because
we try to guess the format and when this fails (because there is no medium) we
exit with an error message.

This patch should restore the old behaviour by assuming raw format for such
drives.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -331,8 +331,8 @@ static BlockDriver *find_image_format(const char *filename)
     if (ret < 0)
         return NULL;
 
-    /* Return the raw BlockDriver * to scsi-generic devices */
-    if (bs->sg) {
+    /* Return the raw BlockDriver * to scsi-generic devices or empty drives */
+    if (bs->sg || !bdrv_is_inserted(bs)) {
         bdrv_delete(bs);
         return bdrv_find_format("raw");
     }