qemu: don't error out when cgroups don't exist

When creating cgroups for vcpu and emulator threads whilst starting a
domain, we explicitly skip creating those cgroups in case priv->cgroup
is NULL (cgroups not supported) because SetAffinity() serves the same
purpose.  If the host supports only some cgroups (the ones we need are
either unmounted or disabled in qemu.conf), we error out with weird
message even though we could continue starting the domain.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1097028Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/qemu/qemu_cgroup.c

@@ -906,6 +906,15 @@ qemuSetupCgroupForVcpu(virDomainObjPtr vm)
         return -1;
     }
 
+    /*
+     * If CPU cgroup controller is not initialized here, then we need
+     * neither period nor quota settings.  And if CPUSET controller is
+     * not initialized either, then there's nothing to do anyway.
+     */
+    if (!virCgroupHasController(priv->cgroup, VIR_CGROUP_CONTROLLER_CPU) &&
+        !virCgroupHasController(priv->cgroup, VIR_CGROUP_CONTROLLER_CPUSET))
+        return 0;
+
     /* We are trying to setup cgroups for CPU pinning, which can also be done
      * with virProcessSetAffinity, thus the lack of cgroups is not fatal here.
      */
@@ -985,6 +994,15 @@ qemuSetupCgroupForEmulator(virQEMUDriverPtr driver,
         return -1;
     }
 
+    /*
+     * If CPU cgroup controller is not initialized here, then we need
+     * neither period nor quota settings.  And if CPUSET controller is
+     * not initialized either, then there's nothing to do anyway.
+     */
+    if (!virCgroupHasController(priv->cgroup, VIR_CGROUP_CONTROLLER_CPU) &&
+        !virCgroupHasController(priv->cgroup, VIR_CGROUP_CONTROLLER_CPUSET))
+        return 0;
+
     if (priv->cgroup == NULL)
         return 0; /* Not supported, so claim success */