hw/audio: replace exit with unrealize in hda_codec_device_class_init

The exit callback of DeviceClass will be removed in the future, so
convert to unrealize in the init functioin

Signed-off-by: Zihan Yang <tgnyang@gmail.com>
Message-id: 1493211188-24086-4-git-send-email-tgnyang@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
