Following the conversation :
My understanding was that the way to use out of the tree vif_driver is
to set vif_driver option in nova.conf
until there is a better way to support such cases but the commit 
removed this option.
Can someone clarify the current status (i.e. what is the current way to
do it ) in Juno?
(libvirt: remove vif_driver config parameter)
OpenStack-dev mailing list