qemu: virtiofs: do not crash if cgroups are missing

On domain startup, qemuSetupCgroupForExtDevices checks
if a cgroup controller is present and skips the setup if not.

Add a similar check to qemuVirtioFSSetupCgroup to prevent
crashing when hotplugging a virtiofs filesystem.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_virtiofs.c

@@ -353,6 +353,9 @@ qemuVirtioFSSetupCgroup(virDomainObj *vm,
     pid_t pid = -1;
     int rc;
 
+    if (!cgroup)
+        return 0;
+
     if (!(pidfile = qemuVirtioFSCreatePidFilename(vm, fs->info.alias)))
         return -1;