isa: QOM'ify isa_bus_from_device()

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

hw/isa.h

@@ -82,7 +82,7 @@ void isa_register_portio_list(ISADevice *dev, uint16_t start,
 
 static inline ISABus *isa_bus_from_device(ISADevice *d)
 {
-    return DO_UPCAST(ISABus, qbus, d->qdev.parent_bus);
+    return ISA_BUS(qdev_get_parent_bus(DEVICE(d)));
 }
 
 extern hwaddr isa_mem_base;