\documentclass[a4paper]{report}
\usepackage{listings}

\newcommand{\xmlcode}[1]{
{\lstset{frameround=tttt,language=XML, basicstyle=\footnotesize, stepnumber=1, stringstyle={\ttfamily}, frame=single, breaklines=true, showspaces=false, showstringspaces=false, numbers=left}
\begin{lstlisting}
#1
\end{lstlisting}
}}

\begin{document}

This is a test

\xmlcode{
<foo>
    <bar />
</foo>
}

\end{document}