qemu: Fix starting domains with no cpu cgroup

If cgroups are enabled in general but cpu cgroup is disabled in
qemu.conf or not mounted at all, libvirt would refuse to start any
domain even though scheduler parameters are not set in domain XML.

This patch makes cpu cgroup mandatory only for domains that actually
want to use it.

src/qemu/qemu_cgroup.c

@@ -549,7 +549,8 @@ int qemuSetupCgroupForVcpu(struct qemud_driver *driver, virDomainObjPtr vm)
     if (driver->cgroup == NULL)
         return 0; /* Not supported, so claim success */
 
-    if (!qemuCgroupControllerActive(driver, VIR_CGROUP_CONTROLLER_CPU)) {
+    if ((period || quota) &&
+        !qemuCgroupControllerActive(driver, VIR_CGROUP_CONTROLLER_CPU)) {
         virReportError(VIR_ERR_SYSTEM_ERROR, "%s",
                        _("cgroup cpu is not active"));
         return -1;