qemu: Don't fail startup/attach for IOThreads if no JSON

If the qemu being used doesn't support JSON, then querying for IOThread
data would fail. In that case, ensure the *iothreads is NULL and return 0
as the count of iothreads available.

src/qemu/qemu_monitor.c

@@ -4112,10 +4112,10 @@ qemuMonitorGetIOThreads(qemuMonitorPtr mon,
         return -1;
     }
 
+    /* Requires JSON to make the query */
     if (!mon->json) {
-        virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
-                       _("JSON monitor is required"));
-        return -1;
+        *iothreads = NULL;
+        return 0;
     }
 
     return qemuMonitorJSONGetIOThreads(mon, iothreads);