xen gntdev: use gnttab_map_refs and gnttab_unmap_refs

Use gnttab_map_refs and gnttab_unmap_refs to map and unmap the grant
ref, so that we can have a corresponding struct page.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>