Udev is a dependency of xorg and other development packages that breaks OpenVZ containers if installed or upgraded.
Re-create the missing devices after an upgrade via:
vzctl exec {VEID} /sbin/MAKEDEV tty
vzctl exec {VEID} /sbin/MAKEDEV ptyFor a permanent fix, edit /etc/rc.sysinit to disable udev and auto-repair the devices:
#/sbin/start_udev
/sbin/MAKEDEV tty
/sbin/MAKEDEV pty