block: Add blk_commit_all()

Later, we will remove bdrv_commit_all() and move its contents here, and
in order to replace bdrv_commit_all() calls by calls to blk_commit_all()
before doing so, we need to add it as an alias now.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-backend.c

@@ -1329,3 +1329,8 @@ BlockBackendRootState *blk_get_root_state(BlockBackend *blk)
 {
     return &blk->root_state;
 }
+
+int blk_commit_all(void)
+{
+    return bdrv_commit_all();
+}