xen: use system ipxe instead of internal one

## Issue description

Currently, the xen pulls its own instance of ipxe: [pkgs/applications/virtualization/xen/4.5.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/virtualization/xen/4.5.nix#L134).

It would be good to use the system ipxe, and/or to make it optional, as mentioned in #26492.


## Technical details

* System: `17.09.git.c38a36f (Hummingbird)`
