paaudio: wait until the playback stream is ready

Don't call pa_stream_writable_size() in qpa_get_buffer_out()
before the playback stream is ready. This prevents a lot of the
following pulseaudio error messages.

pulseaudio: pa_stream_writable_size failed
pulseaudio: Reason: Bad state

To reproduce start qemu with
-parallel none -device gus,audiodev=audio0 -audiodev pa,id=audio0

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-id: 9315afe5-5958-c0b4-ea1e-14769511a9d5@t-online.de
Message-Id: <20210110100239.27588-15-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>