console: kill DisplayState->opaque

It's broken by design.  There can be multiple DisplayChangeListener
instances, so they simply can't store state in the (single) DisplayState
struct.  Try 'qemu -display gtk -vnc :0', watch it crash & burn.

With DisplayChangeListenerOps having a more sane interface now we can
simply use the DisplayChangeListener pointer to get access to our
private data instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
