(maint) Remove duplication from virtual fact

Prior to this commit the SunOS and OpenBSD resolutions for the virtual
fact had duplicate logic. This commit abstracts the logic into the
`Facter::Util::Virtual.parse_virtualization` helper.
Merge pull request #444 from ajroetker/maint/master/remove_duplication_from_virtual_fact

(maint) Remove duplication from virtual fact