blockdev: check for BLOCK_OP_TYPE_INTERNAL_SNAPSHOT_DELETE

The BLOCK_OP_TYPE_INTERNAL_SNAPSHOT_DELETE op blocker exists but was
never used!  Let's fix that so snapshot delete can be blocked.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -1132,6 +1132,10 @@ SnapshotInfo *qmp_blockdev_snapshot_delete_internal_sync(const char *device,
     aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(aio_context);
 
+    if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_INTERNAL_SNAPSHOT_DELETE, errp)) {
+        goto out_aio_context;
+    }
+
     ret = bdrv_snapshot_find_by_id_and_name(bs, id, name, &sn, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);