cgroups: fix braino during controller list creation

Co-mounted controllers are conventionally separated by ",".

Fixes: https://jenkins.linuxcontainers.org/job/lxd-github-commit/1905/arch=amd64,backend=dir,compiler=golang-1.15/consoleFull
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/cgroups/cgfsng.c

@@ -337,7 +337,7 @@ static char **list_add_controllers(char *controllers)
 	__do_free_string_list char **list = NULL;
 	char *it;
 
-	lxc_iterate_parts(it, controllers, " \t\n") {
+	lxc_iterate_parts(it, controllers, ", \t\n") {
 		int ret;
 
 		ret = list_add_string(&list, it);