xen/PVH: Set up GS segment for stack canary

We are making calls to C code (e.g. xen_prepare_pvh()) which may use
stack canary (stored in GS segment).

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