usb descriptors: add settable strings.

This patch allows to set usb descriptor strings per device instance.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/hw/usb-bus.c b/hw/usb-bus.c
index 8b4583c..af537c2 100644
--- a/hw/usb-bus.c
+++ b/hw/usb-bus.c
@@ -48,6 +48,7 @@
     pstrcpy(dev->product_desc, sizeof(dev->product_desc), info->product_desc);
     dev->info = info;
     dev->auto_attach = 1;
+    QLIST_INIT(&dev->strings);
     rc = dev->info->init(dev);
     if (rc == 0 && dev->auto_attach)
         usb_device_attach(dev);