audio/intel-hda: Fix Inheritance hierachy

audio/intel-hda: Fix Inheritance hierachy

The ich6 and ich9 variants either need to inherit one from the other,
or both from a common base class, otherwise its not possible to create
a QOM cast macro for use by the shared implementation functions.
Went for option B, with a common base class.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 54453b6aa8afa1a76b2ec1932f1d7fd25205d0bc.1370496582.git.peter.crosthwaite@xilinx.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>