qapi: qapi for audio backends

qapi: qapi for audio backends

This patch adds structures into qapi to replace the existing
configuration structures used by audio backends currently. This qapi
will be the base of the -audiodev command line parameter (that replaces
the old environment variables based config).

This is not a 1:1 translation of the old options, I've tried to make
them much more consistent (e.g. almost every backend had an option to
specify buffer size, but the name was different for every backend, and
some backends required usecs, while some other required frames, samples
or bytes). Also tried to reduce the number of abbreviations used by the
config keys.

Some of the more important changes:
* use `in` and `out` instead of `ADC` and `DAC`, as the former is more
  user friendly imho
* moved buffer settings into the global setting area (so it's the same
  for all backends that support it. Backends that can't change buffer
  size will simply ignore them). Also using usecs, as it's probably more
  user friendly than samples or bytes.
* try-poll is now an alsa backend specific option (as all other backends
  currently ignore it)

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 5461b514dbf3e0bc31b0abb6498a9b3a008c271e.1552083282.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>