storage: add overlay as valid backend

src/lxc/storage/storage_utils.c

@@ -445,6 +445,7 @@ bool is_valid_storage_type(const char *type)
 	    strcmp(type, "loop") == 0 ||
 	    strcmp(type, "lvm") == 0 ||
 	    strcmp(type, "nbd") == 0 ||
+	    strcmp(type, "overlay") == 0 ||
 	    strcmp(type, "overlayfs") == 0 ||
 	    strcmp(type, "rbd") == 0 ||
 	    strcmp(type, "zfs") == 0)