vircgroup: one central point for adding tasks to cgroups

Use virCgroupAddTaskController in virCgroupAddTask so we have one
single point where we add tasks to cgroups.
Signed-off-by: NHenning Schild <henning.schild@siemens.com>

src/util/vircgroup.c

@@ -1183,7 +1183,7 @@ virCgroupAddTask(virCgroupPtr group, pid_t pid)
         if (i == VIR_CGROUP_CONTROLLER_SYSTEMD)
             continue;
 
-        if (virCgroupSetValueU64(group, i, "tasks", pid) < 0)
+        if (virCgroupAddTaskController(group, pid, i) < 0)
             goto cleanup;
     }