Vivid switched from upstart to systemd. I used to have xfvb configured to run as a service under updstart. That has now broken.
Under upstart, I had /etc/init/xvfb.conf:
description "Xvfb X Server" start on (net-device-up and local-filesystems and runlevel ) stop on runlevel  exec /usr/bin/Xvfb :99 -screen 0 1024x768x24
and I could start it with
sudo service xvfb start
Under 15.04, I now get this message when I try to use service to start xvfb:
Failed to start xvfb.service: Unit xvfb.service failed to load: No such file or directory.
What is the new way to configure xvfb to run as a service with systemd?