docs: Document libxl.conf location

This is similar to my earlier commit which documented lxc.conf
location. Just like LXC, the libxl driver has only the system
connection and thus only few places need changing.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>

docs/drvxen.html.in

@@ -50,6 +50,17 @@ xen+tcp://example.com/system      (remote access, SASl/Kerberos)
 xen+ssh://root@example.com/system (remote access, SSH tunnelled)
 </pre>
 
+
+    <h2><a id="configFiles">Location of configuration files</a></h2>
+
+    <p>
+      The libxl driver comes with sane default values. However, during its
+      initialization it reads a configuration file which offers system
+      administrator to override some of that default. The file is located
+      under <code>/etc/libvirt/libxl.conf</code>
+    </p>
+
+
     <h2><a id="imex">Import and export of libvirt domain XML configs</a></h2>
 
     <p>