x86/setup: Don't skip 2MiB underneath relocated Xen image

x86/setup: Don't skip 2MiB underneath relocated Xen image

Set 'e' correctly to reflect the location that Xen is actually relocated
to from its default 2MiB location. Not 2MiB below that.

This is only vaguely a bug fix. The "missing" 2MiB would have been used
in the end, and fed to the allocator. It's just that other things don't
get to sit right up *next* to the Xen image, and it isn't very tidy.

For live update, I'd quite like a single contiguous region for the
reserved bootmem and Xen, allowing the 'slack' in the former to be used
when Xen itself grows larger. Let's not allow 2MiB of random heap pages
to get in the way...

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>