audio: basic support for multi backend audio

Audio functions no longer access glob_audio_state, instead they get an
AudioState as a parameter.  This is required in order to support
multiple backends.

glob_audio_state is also gone, and replaced with a tailq so we can store
more than one states.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: 67aef54f9e729a7160fe95c465351115e392164b.1566168923.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
7 files changed