audio: Don't free hw resources until after hw backend is stopped

When stopping an audio voice, call the audio backend's fini
method before calling audio_pcm_hw_free_resources_ rather than
afterwards. This allows backends which use helper threads (like
pulseaudio) to terminate those threads before the conv_buf or
mix_buf are freed and avoids race conditions where the helper
may access a NULL pointer or freed memory.

Cc: qemu-stable@nongnu.org
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1418406239-9838-1-git-send-email-peter.maydell@linaro.org