Improve OpenVZ.pm

* Output now matches the regular interface plugin
  * before it was if_octets -> lo -> tx, now it is lo -> if_octets -> tx
* Disables CPU plugin by default, as all guests would report the same info give or take a few  milliseconds
  * with 32 core machines with 8 guests, that was a lot of duplicate data

Sample Graphite output can be seen here,
http://snag.gy/CXyUJ.jpg

Target was:
`render?from=-1h&target=test-vps_chef_blueboxgrid_com.interface.venet0.*.*&width=1200&height=800`