ide/piix: QOM casting sweep

Use standard QOM cast macro. Remove usage of DO_UPCAST() and
direct -> style upcasting.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
ide: Introduce abstract QOM type for PCIIDEState

Needed for QOM casts.

Signed-off-by: Andreas Färber <afaerber@suse.de>