x86/EFI: avoid Xen image when looking for module/kexec position

x86/EFI: avoid Xen image when looking for module/kexec position

When booting straight from EFI, we don't further try to relocate Xen.
As a result, so far we also didn't avoid the area Xen uses when looking
for a location to put modules or the kexec area. Introduce a fake
module slot to deal with that without having to fiddle with a lot of
code.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>