Check if VTX is enabled on hypervisor nodes

VTX is required when using KVM as hypervisor on compute nodes.

When the fact will be merged upstream, the lib/facter/vtx.rb file will be
dropped to use the one from facter-core.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #611 from enovance/vtx-check

Check if VTX is enabled on hypervisor nodes