usb-bus: Don't detach non attached devices on device exit

This causes an "Error: tried to detach unattached usb device " to be printed,
this can happen when deleting ie a usb host qdev, which did not
get attached (because a device matching the filter never got plugged in).
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb-bus.c

@@ -84,7 +84,9 @@ static int usb_qdev_exit(DeviceState *qdev)
     USBDevice *dev = DO_UPCAST(USBDevice, qdev, qdev);
     USBBus *bus = usb_bus_from_device(dev);
 
-    usb_device_detach(dev);
+    if (dev->attached) {
+        usb_device_detach(dev);
+    }
     bus->ops->device_destroy(bus, dev);
     if (dev->info->handle_destroy) {
         dev->info->handle_destroy(dev);