[virtual] improve detection of proxmox-based KVM virtual machines

This small change allows detection of proxmox-based (proxmox is set of tools around KVM hypervisor to make it more "enterprise" - live migration, web interface for configuring net/storage/etc, HA, etc). 
Unfortunately signatures included in facter ATM cannot detect those VMs, hence they left as virtual => physical. 
Test dmidecode from one of those VMs: https://gist.github.com/rlex/5291283
cpuinfo from same machine: https://gist.github.com/rlex/5291293