(#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 ' |grep ^virt-what:' can be removed from the virt-what
command in Facter::Util::Virtual.

A virt-what fix was submitted at https://bugzilla.redhat.com/719611.

I am not sure that the test for this commit is correct.  I don't have an environment setup for running the tests.  Apologies for that, but hopefully the fix is obviously correct and someone can help out with the test if it is not sufficient.