(#8210) Add virt-what as a dependency

The pull request at https://github.com/puppetlabs/facter/pull/340 introduced detecting the virtualization type with virt-what, which is much more reliable than facter's own techniques. This pull requests adds a dependency on virt-what since it's included in Debian at least since  squeeze and in Ubuntu since lucid.

Sorry, I accidently based the other pull request on the master branch.