xen: mark unsupported versions as vulnerable
xen: mark unsupported versions as vulnerable

Our support for Xen lacks maintenance and since Xen has monthly security
advisories it is reasonable to assume our version is affected by a
multitude of security problems that are fixed upstream.

How many advisories? Browsing oss-security shows the following number of
advisories in each of the following years:

2022: 3
2021: 53
2020: 54
2019: 46 <-- we are *here*

https://xenbits.xen.org/docs/unstable/support-matrix.html

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs