(#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.

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 committed upstream¹.

Thanks to Adrien Thebo for help on making this testable.

¹ http://git.annexia.org/?p=virt-what.git;a=commitdiff;h=2f47e06
Merge branch 'pull-413' into 1.7.x
Merge branch '1.7.x'
[virtual] improve detection of proxmox-based KVM virtual machines
[virtual] rewrite cpu-based detection of KVM-based VMs
[virtual] add tests for dmidecode / bochs and lspci / virtio
[virtual] use manufacturer instead of product name in Bochs/KVM detection
Merge branch 'pull-419' into 1.7.x
Merge branch '1.7.x'
Revert "Merge branch 'pull-419' into 1.7.x"

Facter 1.7 is in RC; this is being reverted to avoid delaying 1.7.0.

This reverts commit 229fca73ac31e33534c8be3e0322345a3b07cc95, reversing
changes made to b7784a3af80c70a833dfd3de1fa32dc1eb62384e.
Merge branch '1.7.x'
Revert "Revert "Merge branch 'pull-419' into 1.7.x""

This revert serves to move GH-419 into master.

This reverts commit 08188324fe44efc5e93defb2ee49955d825f3a02.