xen: remove other open-coded use of libxengnttab

xen: remove other open-coded use of libxengnttab

Now that helpers are available in xen_backend, use them throughout all
Xen PV backends.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony Perard <anthony.perard@citrix.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Greg Kurz <groug@kaod.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
v2:
 - New in v2