mips malta: Fix fdc regression and use qdev for i8042 setup

mips malta: Fix fdc regression and use qdev for i8042 setup

8baf73adf664e79eae201c3f618078a220a661d9 (qdev/isa: convert fdc)
breaks MIPS Malta:

Tried to create isa device isa-fdc with no isa bus present

Fix this by creating an isa bus for piix4.
This change also requires some more qdev related changes
(similar changes were applied to pc.c) and allows
cleaning of piix3/piix4 code.

Thanks to Gerd Hoffmann for his hints.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>