spapr: Add DRC release method

At the moment, spapr_drc_release() has an ugly switch on the DRC type to
call the right, device-specific release function.  This cleans it up by
doing that via a proper QOM method.

It's still arguably an abstraction violation for the DRC code to call into
the specific device code, but one mess at a time.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
