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 <alistair.francis@xilinx.com>
Acked-by: John Snow <jsnow@redhat.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
