ahci.c: Don't assume AHCIState's parent is AHCIPCIState

ahci.c: Don't assume AHCIState's parent is AHCIPCIState

The AHCIState struct can either have AHCIPCIState or SysbusAHCIState
as a parent. The ahci_irq_lower() and ahci_irq_raise() functions
assume that it is always AHCIPCIState, which is not always the
case, which causes a seg fault. Verify what the container of AHCIState
is before setting the PCIDevice struct.

Signed-off-by: Alistair Francis <[email protected]>
Acked-by: John Snow <[email protected]>
Reviewed-by: Peter Crosthwaite <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>