If you want to use a physical device in your VE, you can use this simple command:
# vzctl set <VEID> --netdev_add <network_device>
replace <VEID> with container ID and <network_device> with your physical network device you want to use, such as eth0
WARNING: don't try this command in remote mode, if you are trying it at the first time! You can lose remote access due to strange logic of mapping: this method fully removes given ethernet device from host system! So, don't give to a guest system your primary uplink interface.