add myself as reviewer for Xen support in Linux

It would be good for me to keep an eye on the patches that touch Xen
support in Linux to try to spot changes that break Xen on ARM early on.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>