audio: omitting audiodev= parameter is only deprecated

Unfortunately, changes introduced in af2041ed "audio: audiodev=
parameters no longer optional when -audiodev present" breaks backward
compatibility.  This patch changes the error into a deprecation warning.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: 02d4328c33455742d01e0b62395013e95293c3ba.1566847960.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/audio.c

@@ -1412,8 +1412,9 @@ static AudioState *audio_init(Audiodev *dev, const char *name)
         drvname = AudiodevDriver_str(dev->driver);
     } else if (!QTAILQ_EMPTY(&audio_states)) {
         if (!legacy_config) {
-            dolog("You must specify an audiodev= for the device %s\n", name);
-            exit(1);
+            dolog("Device %s: audiodev default parameter is deprecated, please "
+                  "specify audiodev=%s\n", name,
+                  QTAILQ_FIRST(&audio_states)->dev->id);
         }
         return QTAILQ_FIRST(&audio_states);
     } else {
@@ -1548,8 +1549,7 @@ CaptureVoiceOut *AUD_add_capture(
 
     if (!s) {
         if (!legacy_config) {
-            dolog("You must specify audiodev when trying to capture\n");
-            return NULL;
+            dolog("Capturing without setting an audiodev is deprecated\n");
         }
         s = audio_init(NULL, NULL);
     }

qemu-deprecated.texi

@@ -72,6 +72,13 @@ backend settings instead of environment variables.  To ease migration to
 the new format, the ``-audiodev-help'' option can be used to convert
 the current values of the environment variables to ``-audiodev'' options.
 
+@subsection Creating sound card devices and vnc without audiodev= property (since 4.2)
+
+When not using the deprecated legacy audio config, each sound card
+should specify an @code{audiodev=} property.  Additionally, when using
+vnc, you should specify an @code{audiodev=} propery if you plan to
+transmit audio through the VNC protocol.
+
 @subsection -mon ...,control=readline,pretty=on|off (since 4.1)
 
 The @code{pretty=on|off} switch has no effect for HMP monitors, but is