(FACT-3452) Make Xen resolver more strict

[mhashizume]

Prior to this commit, Facter could misidentify non-Xen systems as being Xen-based.

In some circumstances, a non-Xen system will have /dev/xvd* files present as symlinks to other devices. This was observed on an Amazon Linux 2023 machine, which was running on a Nitro (KVM-based) hypervisor but symlinked /dev/xvda1 to an nvme device.

This commit updates the Xen resolver to check if /dev/xvda1 is a symlink to avoid misidentifying if a system is Xen-based.

[mhashizume]

On an Amazon Linux 2023 machine running on a t4g.medium instance on Nitro, the system symlinks /dev/xvda1:

$ ls -l /dev/xvda1
lrwxrwxrwx. 1 root root 9 Dec  4 15:58 /dev/xvda1 -> nvme0n1p1

This is how Facter currently behaves:

$ git describe
4.2.5-320-g421f9731c
$ bundle exec facter cloud

$ bundle exec facter virtual
xenu

Note that Facter doesn't resolve the cloud fact and misidentifies the machine as Xen.

With this PR:

$ git describe
4.2.5-375-ga11f4f8a4
$ bundle exec facter cloud
{
  provider => "aws"
}
$ bundle exec facter virtual
kvm