[RFE] Remove Xen Grub and Xen PV(H) support

[jepio]

Current situation

The Flatcar kernel is compressed with ZSTD since commit:

commit 29f2903c8795d3269e1ffff507190c82e01267f2
Author: Kai Lüke <kailuke@microsoft.com>
Date:   Mon Jul 26 14:29:11 2021 +0200

    sys-kernel/coreos-modules: switch to gzip/zstd compression

When booting a Xen domU in pv/pvh mode pvgrub/pvgrub2/pygrub only support uncompressed or xz compressed kernels. This means the support for pv/pvh booting has been broken for at almost 3 years and no one noticed.

We can remove some files from /boot and make the kernel a bit smaller if we say goodbye to xen grub.

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Impact

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Ideal future situation

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Implementation options

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Additional information

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[jepio]

According to some comments - pvh grub can load zstd compressed kernels. just need to figure out if it can chainload.

[jepio]

It doesn't seem to support grub->grub chainloading but pygrub->grub works: flatcar/scripts#1879