Bug#1040767: facter: inconsistent detection of Xen dom0

[Sergio Gelato]

systemd has a similar issue, tracked in #1038901. (Maybe one should ask the 
virt-what maintainers whether they agree with 
https://github.com/systemd/systemd/issues/28113#issuecomment-1621986461, in 
which case this bug can be reassigned to virt-what.)