hw/audio: QOM'ify milkymist-ac97.c

* Drop the old SysBus init function and use instance_init
* Move AUD_open_in / AUD_open_out function into realize stage

Acked-by: Michael Walle <michael@walle.cc>
Tested-by: Michael Walle <michael@walle.cc>
Signed-off-by: xiaoqiang zhao <zxq_yx_007@163.com>
Message-id: 1463111220-30335-5-git-send-email-zxq_yx_007@163.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>