tdm_client: Introduce 'tdm_virtual_output' on the client side.

tdm_client: Introduce 'tdm_virtual_output' on the client side.

NOTE: This needs extra implementation of server side to be operated properly.

APIs of 'tdm_virtual_output' will provide services to applications like
Wi-Fi display manager. Any application which has proper privilege can ask
tdm_server for considering itself as a sort of virtual output.
Once an application acquires the instance of 'tdm_virtual_output' successfully,
server considers it as a normal output.

Change-Id: Ib6085db09521facd90d28637056b48e166f8a90e
Signed-off-by: Junkyeong Kim <jk0430.kim@samsung.com>