(#8210) Add virt-what dependency in Debian packaging

The pull request at puppetlabs/facter#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 in the debian
packaging since virt-what is included in Debian at least since squeeze
and in Ubuntu since lucid and has negligible overhead.