This peels off a patches from the top of another longer series
where Borislav suggested actually fixing a problem rather than
complaining about it:

	https://lore.kernel.org/all/20240307162752.GAZenrCDqs0lMTjmu1@fat_crate.local/

There's a little detour here to explain some mysterious (to me)
CPUID prodding.

The end result gets rid of some fragile Xen boot code.  It also clears
the way for a future series to bring some order to the early boot code
and remove some of the random tinkering that is there today.