Disable use of 'reason' field in block IO event in QEMU

QEMU upstream decided against adding a 'reason' field to
the block IO event in QMP. Disable this code to remove a
annoying warning message. It will be renabled when the
error string reason is re-introduced in QEMU

src/qemu/qemu_monitor_json.c

@@ -589,10 +589,14 @@ static void qemuMonitorJSONHandleIOError(qemuMonitorPtr mon, virJSONValuePtr dat
         VIR_WARN0("missing device in disk io error event");
     }
 
+#if 0
     if ((reason = virJSONValueObjectGetString(data, "reason")) == NULL) {
         VIR_WARN0("missing reason in disk io error event");
         reason = "";
     }
+#else
+    reason = "";
+#endif
 
     if ((actionID = qemuMonitorIOErrorActionTypeFromString(action)) < 0) {
         VIR_WARN("unknown disk io error action '%s'", action);