Avoid warnings when virt-what produces no output
    
    The previous patch caused spurious warnings from facter when the
    virt-what command produced no output, as it would if run by root on a
    physical host:
    
    $ sudo facter virtual
    Could not retrieve virtual: private method `gsub' called for nil:NilClass
    physical
    
    Josh Cooper from Puppet Labs caught and fixed this.