block: Catch backing files assigned to non-COW drivers

Since we parse backing.* options to add a backing file from the command
line when the driver didn't assign one, it has been possible to have a
backing file for e.g. raw images (it just was never accessed).

This is obvious nonsense and should be rejected.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/block/block_int.h

@@ -100,6 +100,9 @@ struct BlockDriver {
      */
     bool bdrv_needs_filename;
 
+    /* Set if a driver can support backing files */
+    bool supports_backing;
+
     /* For handling image reopen for split or non-split files */
     int (*bdrv_reopen_prepare)(BDRVReopenState *reopen_state,
                                BlockReopenQueue *queue, Error **errp);