(#19989) Filter virt-what warnings from virtual fact

Presently, the virt-what command puts errors on stdout, prefixed with
virt-what:.  This ends up in the virtual fact, which is undesirable.

If/when a fixed virt-what is released and available for supported
systems, the 'output.gsub /^virt-what: .*$/' can be removed from
Facter::Util::Virtual::virt_what().

A virt-what fix was submitted at https://bugzilla.redhat.com/719611.

Thanks to Adrien Thebo for help on making this testable.