audio: second batch of -audiodev support, adding support for multiple backends.

# gpg: Signature made Wed 21 Aug 2019 09:40:37 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>" [full]
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>" [full]
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>" [full]
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/audio-20190821-pull-request:
  audio: fix memory leak reported by ASAN
  audio: use size_t where makes sense
  audio: remove read and write pcm_ops
  paaudio: fix playback glitches
  audio: do not run each backend in audio_run
  audio: remove audio_MIN, audio_MAX
  paaudio: properly disconnect streams in fini_*
  paaudio: do not move stream when sink/source name is specified
  audio: audiodev= parameters no longer optional when -audiodev present
  paaudio: prepare for multiple audiodev
  audio: add audiodev properties to frontends
  audio: add audiodev property to vnc and wav_capture
  audio: basic support for multi backend audio
  audio: reduce glob_audio_state usage
  audio: Add missing fall through comments

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>