vircgroup: register cgroup v2 backend

All mandatory callbacks are implemented for cgroup v2 backend so we
can register it now.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/util/vircgroupbackend.c

@@ -52,6 +52,7 @@ virCgroupBackendRegister(virCgroupBackendPtr backend)
 static void
 virCgroupBackendOnceInit(void)
 {
+    virCgroupV2Register();
     virCgroupV1Register();
 }