This will allow us to disable mixeng when we use a decent backend.

Disabling mixeng have a few advantages:
* we no longer convert the audio output from one format to another, when
  the underlying audio system would just convert it to a third format.
  We no longer convert, only the underlying system, when needed.
* the underlying system probably has better resampling and sample format
  converting methods anyway...
* we may support formats that the mixeng currently does not support (S24
  or float samples, more than two channels)
* when using an audio server (like pulseaudio) different sound card
  outputs will show up as separate streams, even if we use only one
  backend

Disadvantages:
* audio capturing no longer works (wavcapture, and vnc audio extension)
* some backends only support a single playback stream or very picky
  about the audio format.  In this case we can't disable mixeng.

However mixeng is not removed, only made optional, so this shouldn't be
a big concern.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
---

Notes:
    Changes from v1:
    
    * renamed mixeng to mixing-engine

 qapi/audio.json | 5 +++++
 qemu-options.hx | 6 ++++++
 2 files changed, 11 insertions(+)

diff --git a/qapi/audio.json b/qapi/audio.json
index 9fefdf5186..0535eff794 100644
--- a/qapi/audio.json
+++ b/qapi/audio.json
@@ -11,6 +11,10 @@
 # General audio backend options that are used for both playback and
 # recording.
 #
+# @mixing-engine: use QEMU's mixing engine to mix all streams inside QEMU. When
+#                 set to off, fixed-settings must be also off. Not every backend
+#                 compatible with the off setting (default on, since 4.2)
+#
 # @fixed-settings: use fixed settings for host input/output. When off,
 #                  frequency, channels and format must not be
 #                  specified (default true)
@@ -31,6 +35,7 @@
 ##
 { 'struct': 'AudiodevPerDirectionOptions',
   'data': {
+    '*mixing-engine':  'bool',
     '*fixed-settings': 'bool',
     '*frequency':      'uint32',
     '*channels':       'uint32',
diff --git a/qemu-options.hx b/qemu-options.hx
index bbfd936d29..734a3b5d65 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -433,6 +433,7 @@ DEF("audiodev", HAS_ARG, QEMU_OPTION_audiodev,
     "                specifies the audio backend to use\n"
     "                id= identifier of the backend\n"
     "                timer-period= timer period in microseconds\n"
+    "                in|out.mixing-engineeng= use mixing engine to mix streams inside QEMU\n"
     "                in|out.fixed-settings= use fixed settings for host audio\n"
     "                in|out.frequency= frequency to use with fixed settings\n"
     "                in|out.channels= number of channels to use with fixed settings\n"
@@ -503,6 +504,11 @@ Identifies the audio backend.
 Sets the timer @var{period} used by the audio subsystem in microseconds.
 Default is 10000 (10 ms).
 
+@item in|out.mixing-engine=on|off
+Use QEMU's mixing engine to mix all streams inside QEMU.  When off,
+@var{fixed-settings} must be off too.  Not every backend is fully
+compatible with the off setting.  Default is on.
+
 @item in|out.fixed-settings=on|off
 Use fixed settings for host audio.  When off, it will change based on
 how the guest opens the sound card.  In this case you must not specify
-- 
2.23.0