hw/xen: Add "mode" parameter to xen-block devices

Block devices don't work in PV Grub (0.9x) if there is no mode specified. It
complains: "Error ENOENT when reading the mode"

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20250207143724.30792-2-dwmw2@infradead.org>
Signed-off-by: Anthony PERARD <anthony.perard@vates.tech>

hw/block/xen-block.c

@@ -408,6 +408,8 @@ static void xen_block_realize(XenDevice *xendev, Error **errp)
     }
 
     xen_device_backend_printf(xendev, "info", "%u", blockdev->info);
+    xen_device_backend_printf(xendev, "mode",
+                              (blockdev->info & VDISK_READONLY) ? "r" : "w");
 
     xen_device_frontend_printf(xendev, "virtual-device", "%lu",
                                vdev->number);