cameraif: add ABI for para-virtual camera

cameraif: add ABI for para-virtual camera

This is the ABI for the two halves of a para-virtualized
camera driver which extends Xen's reach multimedia capabilities even
farther enabling it for video conferencing, In-Vehicle Infotainment,
high definition maps etc.

The initial goal is to support most needed functionality with the
final idea to make it possible to extend the protocol if need be:

1. Provide means for base virtual device configuration:
 - pixel formats
 - resolutions
 - frame rates
2. Support basic camera controls:
 - contrast
 - brightness
 - hue
 - saturation
3. Support streaming control

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Juergen Gross <jgross@suse.com>