doing a do-release-upgrade inside an openvz container from Ubuntu intrepid 8.10 to Ubuntu jaunty 9.04 resulted in a network problem. The error was
ifup: failed to open statefile /var/run/network/ifstate: No such file or directory
and had some other errors when trying to manipulate the networking from within the container.
The problem: the missing directory used to be created by /etc/init.d/loopback, and now /lib/udev/rules.d/85-ifupdown.rules is in charge of making it.
The solution: add the following line to /etc/init.d/networking just after line 53 which is start)
[ -d /var/run/network ] || mkdir /var/run/network