piix4: use PCI address space instead of system memory

piix4 is only used on MIPS Malta board, which gives get_system_memory()
to pci_register_bus().
Signed-off-by: NHervé Poussineau <hpoussin@reactos.org>
Signed-off-by: NLeon Alrae <leon.alrae@imgtec.com>

hw/isa/piix4.c

@@ -86,7 +86,7 @@ static int piix4_initfn(PCIDevice *dev)
 {
     PIIX4State *d = DO_UPCAST(PIIX4State, dev, dev);
 
-    isa_bus_new(DEVICE(d), get_system_memory(),
+    isa_bus_new(DEVICE(d), pci_address_space(dev),
                 pci_address_space_io(dev));
     piix4_dev = &d->dev;
     qemu_register_reset(piix4_reset, d);