x86/xen: move xen_reserve_extra_memory()

In preparation for making the function static.

No functional change.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20240725073116.14626-2-roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>