you can use $_SERVER['REQUEST_URI'] and add you domain before it so:
<img src="https://wordpress.stackexchange.com/questions/18401/<?php
global $qrcodetag;
$Path=$_SERVER["REQUEST_URI'];
$URI='http://www.example.com'.$Path;
echo $qrcodetag->getQrCodeUrl($URI,100,'UTF-8','L',4,0);
?>">
and change http://www.example.com to your domain.