v4l2: enumerate output ports and add a property to select input/output

Submitted by Aurélien Zanelli

Link to original bug (#746080)

Description

V4L2 device can have multiple input/output port and they expose them through ENUMINPUT/ENUMOUTPUT ioctls. But currently we only enumerate input whatever device is a capture one or an output one. I think, we should enumerate according to the device type.

Also I think it can be useful to enable user to select which input/output they want to use. To solve this I propose to add a property to v4l2object to let user select input/output.

I found this while playing with Vivid which have multiple input/output port.