Xen enablement

[jpds]

I'd like to use ostree with a series of Xen systems I have, however because Xen needs to boot up its dom0 before then loading Linux - it uses a different set of GRUB configuration:

        multiboot2      /boot/xen-4.14-amd64.gz placeholder   ${xen_rm_opts}
        echo    'Loading Linux 5.10.0-13-amd64 ...'
        module2 /boot/vmlinuz-5.10.0-13-amd64 placeholder root=UUID=14ae83c5-923b-4562-bf52-6365bf88b2e2 ro  quiet
        echo    'Loading initial ramdisk ...'
        module2 --nounzip   /boot/initrd.img-5.10.0-13-amd64

I have looked into adding this myself by adding xen_srcpath and xen_namever to https://github.com/ostreedev/ostree/blob/main/src/libostree/ostree-sysroot-deploy.c - however I'm not sure how to tell a deployment that it's specifically a Xen one to then generate the correct GRUB lines in ostree admin instutil grub2-generate. Should there be something like a ostree admin deploy --xen flag?