privsep will default to invoking privsep-helper directly
via sudo, which won't work for people with a locked down
sudo config. To deal with this we should explicitly
configure the os-vif plugins to use nova-rootwrap for
running privsep-helper. This change makes such a change
for the two official in-tree os-vif plugins.
Change-Id: I3d26251206a57599385f2b9f3e0ef7d91daafe35