vircgroup: fix bug in virCgroupEnableMissingControllers

If we are on host with systemd we need to build cgroup hierarchy
ourselves for controllers that are not managed by systemd.

As a starting parent we need to force root group because
virCgroupMakeGroup() takes that parent in order to inherit values
for cpuset controller.

By default cpuset controller is managed by systemd so we will never
hit the issue but for v2 cgroups we need to use parent cgroup every
time.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/util/vircgroup.c

@@ -1491,7 +1491,7 @@ virCgroupEnableMissingControllers(char *path,
     int ret = -1;
 
     if (virCgroupNew(pidleader,
-                     "",
+                     "/",
                      NULL,
                      controllers,
                      &parent) < 0)