xen: Replace lkml.org links with lore

As started by commit 05a5f51c

 ("Documentation: Replace lkml.org
links with lore"), replace lkml.org links with lore to better use a
single source that's more likely to stay available long-term.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210210234618.2734785-1-keescook@chromium.org
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>