Fix qemu create wizard when iothread is set

The variable to use to check the disk type is confid,
as set in line 16 with:
var confid = me.confid || (values.controller + values.deviceid);

www/manager/qemu/HDEdit.js

@@ -39,7 +39,7 @@ Ext.define('PVE.qemu.HDInputPanel', {
 	    delete me.drive.discard;
 	}
 
-	if (values.iothread && me.confid.match(/^virtio\d+$/)) {
+	if (values.iothread && confid.match(/^virtio\d+$/)) {
 	    me.drive.iothread = 'on';
 	} else {
 	    delete me.drive.iothread;