Also add libvirt to addons because of buggy Travis config parsing...
diff --git a/.travis.yml b/.travis.yml
index 97f0522..88c38dc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -62,6 +62,7 @@
           packages:
             - graphviz
             - gcc
+            - libvirt
   # For now allow failures of all the builds which use lxml
   allow_failures:
     - env: ENV=2.6-lxml