We moved all the PIIX3 southbridge code out of hw/pci-host/piix.c,
it now only contains i440FX northbridge code.
Rename it to match the chipset modelled.
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>