Ran into an issue where an openvz container would start up, but the networking would not. Running in verbose mode I saw

ifup-venet WARNING: Device list is empty

After looking around, searching the web I could not come up with anything. Then I saw the container had an /etc/vz folder. Turned out vzctl had got installed by the user on the openvz container, breaking networking. Removing this and a restart fixed the problem.