hw/pci-host: Rename incorrectly named 'piix' as 'i440fx'

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>
diff --git a/MAINTAINERS b/MAINTAINERS
index e299247..363e72a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1241,7 +1241,7 @@
 S: Supported
 F: include/hw/i386/
 F: hw/i386/
-F: hw/pci-host/piix.c
+F: hw/pci-host/i440fx.c
 F: hw/pci-host/q35.c
 F: hw/pci-host/pam.c
 F: include/hw/pci-host/i440fx.h