Improve openvz detection

When running on openvz, ohai always detects virtualization as lxc.

I would like to propose that since openvz kernels contain cgroup support the priority of detection of lxc and openvz be switched around in virtualization.rb (https://github.com/opscode/ohai/blob/master/lib/ohai/plugins/linux/virtualization.rb)

I'm happy to submit a patch if it helps.