Check if interface.mac is defined

Picked up during review; the value can now be empty (libvirt will add a
mac if it wasn't supplied)

Author: goetzk

templates/vm.xml.j2

@@ -51,7 +51,7 @@
     <interface type='network'>
       <source network='{{ interface.network }}'/>
 {% endif %}
-      {% if interface.mac %}
+      {% if interface.mac is defined %}
       <mac address='{{ interface.mac }}'/>
       {% endif %}
       {# if the network configuration is invalid this can still appear in the xml #}