audio: fix GUS audio playback with out.mixing-engine=off

audio: fix GUS audio playback with out.mixing-engine=off

Fix GUS audio playback with out.mixing-engine=off.

The GUS audio device needs to know the amount of samples to
produce in advance.

To reproduce start qemu with
-parallel none -device gus,audiodev=audio0
-audiodev pa,id=audio0,out.mixing-engine=off

and start the cartoon.exe demo in a FreeDOS guest. The demo file
is available on the download page of the GUSemu32 author.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220923183640.8314-2-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>