xen/virtio: use dom0 as default backend for CONFIG_XEN_VIRTIO_FORCE_GRANT

With CONFIG_XEN_VIRTIO_FORCE_GRANT set the default backend domid to 0,
enabling to use xen_grant_dma_ops for those devices.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/grant-dma-ops.c

@@ -365,6 +365,9 @@ void xen_grant_setup_dma_ops(struct device *dev)
 	if (dev->of_node) {
 		if (xen_dt_grant_init_backend_domid(dev, data))
 			goto err;
+	} else if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT)) {
+		dev_info(dev, "Using dom0 as backend\n");
+		data->backend_domid = 0;
 	} else {
 		/* XXX ACPI device unsupported for now */
 		goto err;