qemu: fix regression with pinning

Commit 4b03d591 changed the pinning
behavior in a way that makes some machines non-startable.

The comment mentioning that we cannot control each vcpu when there is
not VCPU<-> PID mapping available is true, however, this isn't
necessarily an error, because this can be caused by old QEMU without
support for "query-cpus" command as well as a software emulated
machines that don't create more than one process.

src/qemu/qemu_cgroup.c

@@ -567,9 +567,9 @@ int qemuSetupCgroupForVcpu(struct qemud_driver *driver, virDomainObjPtr vm)
         /* If we don't know VCPU<->PID mapping or all vcpu runs in the same
          * thread, we cannot control each vcpu.
          */
-        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
-                        _("Unable to get vcpus' pids."));
-        goto cleanup;
+        VIR_WARN("Unable to get vcpus' pids.");
+        virCgroupFree(&cgroup);
+        return 0;
     }
 
     for (i = 0; i < priv->nvcpupids; i++) {