qemu: unref cfg after TerminateMachine has been called

Commit 4882618ed1 added the code that
requests driver cfg, but forgot to unref it.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

src/qemu/qemu_cgroup.c

@@ -1222,6 +1222,8 @@ qemuRemoveCgroup(virQEMUDriverPtr driver,
             VIR_DEBUG("Failed to terminate cgroup for %s", vm->def->name);
     }
 
+    virObjectUnref(cfg);
+
     return virCgroupRemove(priv->cgroup);
 }