xen: re-introduce support for grant v2 interface

The grant v2 support was removed from the kernel with
commit 438b33c7

 ("xen/grant-table:
remove support for V2 tables") as the higher memory footprint of v2
grants resulted in less grants being possible for a kernel compared
to the v1 grant interface.

As machines with more than 16TB of memory are expected to be more
common in the near future support of grant v2 is mandatory in order
to be able to run a Xen pv domain at any memory location.

So re-add grant v2 support basically by reverting above commit.

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