Fix libvirt xml template

Issue found by the test created in previous commit.

templates/libvirt/xen.xml

@@ -108,7 +108,7 @@
                     {% endif %}
 
                     {% if device.domain %}
-                        <backenddomain name="{{ domain }}" />
+                        <backenddomain name="{{ device.domain }}" />
                     {% endif %}
 
                     {% if device.script %}