xen/PVH: Make GDT selectors PVH-specific

We don't need to share PVH GDT layout with other GDTs, especially
since we now have a PVH-speciific entry (for stack canary segment).

Define PVH's own selectors.

(As a side effect of this change we are also fixing improper
reference to __KERNEL_CS)

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