qemu: Fix iothreads issue

If there are no iothreads, then return from qemuProcessDetectIOThreadPIDs
without error; otherwise, the following occurs:

error: Failed to start domain $dom
error: An error occurred, but the cause is unknown

src/qemu/qemu_process.c

@@ -2110,9 +2110,13 @@ qemuProcessDetectIOThreadPIDs(virQEMUDriverPtr driver,
         goto cleanup;
     niothreads = qemuMonitorGetIOThreads(priv->mon, &iothreads);
     qemuDomainObjExitMonitor(driver, vm);
-    if (niothreads <= 0)
+    if (niothreads < 0)
         goto cleanup;
 
+    /* Nothing to do */
+    if (niothreads == 0)
+        return 0;
+
     if (niothreads != vm->def->iothreads) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("got wrong number of IOThread pids from QEMU monitor. "