Commit id 'ca481a6f' added virNetworkRouteDefFree which may be called
in an error path from lxcAddNetworkRouteDefinition with 'route = NULL'.
So just add the (!def) at the top to resolve.
Moving code for parsing and formatting network routes to
networkcommon_conf helps reusing those routes for domains. The route
definition has been hidden to help reducing the number of unnecessary
checks in the format function.
