dec_pci: QOM'ify DEC 21154 PCI-PCI bridge

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>
diff --git a/hw/dec_pci.h b/hw/dec_pci.h
index 79264ba..17dc0c2 100644
--- a/hw/dec_pci.h
+++ b/hw/dec_pci.h
@@ -3,6 +3,8 @@
 
 #include "qemu-common.h"
 
+#define TYPE_DEC_21154 "dec-21154-sysbus"
+
 PCIBus *pci_dec_21154_init(PCIBus *parent_bus, int devfn);
 
 #endif