usb: Allow overriding of usb_desc at the device level

This allows devices to present a different set of descriptors based on
device properties.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb.h

@@ -230,6 +230,7 @@ struct USBDevice {
     USBEndpoint ep_out[USB_MAX_ENDPOINTS];
 
     QLIST_HEAD(, USBDescString) strings;
+    const USBDesc *usb_desc; /* Overrides class usb_desc if not NULL */
     const USBDescDevice *device;
 
     int configuration;

hw/usb/bus.c

@@ -166,6 +166,9 @@ const char *usb_device_get_product_desc(USBDevice *dev)
 const USBDesc *usb_device_get_usb_desc(USBDevice *dev)
 {
     USBDeviceClass *klass = USB_DEVICE_GET_CLASS(dev);
+    if (dev->usb_desc) {
+        return dev->usb_desc;
+    }
     return klass->usb_desc;
 }