xen/virtio: enable grant based virtio on x86

Use an x86-specific virtio_check_mem_acc_cb() for Xen in order to setup
the correct DMA ops.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com> # common code
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>