Introduce type constant. Introduce cast macro and drop dummy busdev
field used with FROM_SYSBUS() that would've broken SYS_BUS_DEVICE().
Avoid accessing parent fields directly.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>