fn - A function that will be called when a USB device is inserted or removed. The function should accept a single parameter, which is a table containing the following keys:
eventType - A string containing either "added" or "removed" depending on whether the USB device was connected or disconnected
productName - A string containing the name of the device
vendorName - A string containing the name of the device vendor
vendorID - A number containing the Vendor ID of the device
productID - A number containing the Product ID of the device