xen/pvh: Initialize grant table for PVH guests

xen/pvh: Initialize grant table for PVH guests

Like PV guests, PVH does not have PCI devices and therefore cannot
use MMIO space to store grants. Instead it balloons out memory and
keeps grants there.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
OraBug: 26662731

(cherry picked from commit 8613d78ab09900010a5b32e78ff229f551d661d6)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>