DeviceState::parent_bus is document as private and should be accessed
through qdev_get_parent_bus(). Use a DEVICE() cast instead of accessing
ISADevice's qdev field directly. Use ISA_BUS() in place of DO_UPCAST().

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>