qemu: Don't remove the "return 0"

Commit f60a50c795 intended to remove the warning only, but not with
the "return 0" together.

src/qemu/qemu_cgroup.c

@@ -471,6 +471,8 @@ qemuSetupMemoryCgroup(virDomainObjPtr vm)
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("Memory cgroup is not available on this host"));
             return -1;
+        } else {
+            return 0;
         }
     }