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>