usbipd includes exported devices in response to exportable device list.
Exclude exported devices from exportable device list to avoid:

- import requests for devices that are exported only to fail the request.
- revealing devices that are imported by a client to another client.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>