Check if VTX is enabled on hypervisor nodes

VTX is required when using KVM as hypervisor on compute nodes.
As an option (false by default), let this puppet module to validate that
VTX is enabled with a Puppet fact (not upstream yet, that's why it's an
option to manage it or not) and fail if VTX is disabled when running KVM
as libvirt driver in nova-compute.

When the fact will be merged upstream, the $check_vtx parameter will be
dropped.