ac97: register reset via qom

So it gets properly unregistered on hot-unplug.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Merge remote-tracking branch 'remotes/kraxel/tags/pull-audio-20140930-1' into staging

ac97: register reset via qom

# gpg: Signature made Tue 30 Sep 2014 12:32:29 BST using RSA key ID D3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"

* remotes/kraxel/tags/pull-audio-20140930-1:
  ac97: register reset via qom

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>