block: early check for blockers on drive-mirror

Even if an op blocker is present for BLOCK_OP_TYPE_MIRROR_SOURCE,
it is checked a bit late and the result is that the target is
created even if drive-mirror subsequently fails.  Add an early
check to avoid this.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -3569,6 +3569,11 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
         return;
     }
 
+    /* Early check to avoid creating target */
+    if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_MIRROR_SOURCE, errp)) {
+        return;
+    }
+
     aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(aio_context);