Fixes #6444 - add support for virtual NICs

Ugh, custom facts, we shouldn't be carrying those.  Could we at least ensure there's a ticket in Facter's tracker to add them there, then non-Discovery users get the benefit of this code too.

Having code in core for additions in Discovery isn't nice, so if there's any way we can extend the Puppet fact parser in the plugin to implement this instead, that would be preferable.