xen/sndif: add sound-device ABI

Add ABI for the two halves of a para-virtualized
sound driver to communicate with each other.

The ABI allows implementing audio playback and capture as
well as volume control and possibility to mute/unmute
audio sources.

Note: depending on the use-case backend can expose more sound
cards and PCM devices/streams than the underlying HW physically
has by employing SW mixers, configuring virtual sound streams,
channels etc. Thus, allowing fine tunned configurations per
frontend.

Signed-off-by: Oleksandr Andrushchenko <[email protected]>
Signed-off-by: Oleksandr Grytsov <[email protected]>
Signed-off-by: Oleksandr Dmytryshyn <[email protected]>
Signed-off-by: Iurii Konovalenko <[email protected]>
Acked-by: Juergen Gross <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>