hw/ide/ahci: Add a Kconfig switch for the AHCI-ICH9 device

hw/ide/ahci: Add a Kconfig switch for the AHCI-ICH9 device

Some of our machines (like the ARM cubieboard) use CONFIG_AHCI for an AHCI
sysbus device, but do not use CONFIG_PCI since they do not feature a PCI
bus. With CONFIG_AHCI but without CONFIG_PCI, currently linking fails:

    ../hw/ide/ich.o: In function `pci_ich9_ahci_realize':
    hw/ide/ich.c:124: undefined reference to `pci_allocate_irq'
    hw/ide/ich.c:126: undefined reference to `pci_register_bar'
    hw/ide/ich.c:128: undefined reference to `pci_register_bar'
    hw/ide/ich.c:131: undefined reference to `pci_add_capability'
    hw/ide/ich.c:147: undefined reference to `msi_init'
    ../hw/ide/ich.o: In function `pci_ich9_uninit':
    hw/ide/ich.c:158: undefined reference to `msi_uninit'
    ../hw/ide/ich.o:(.data.rel+0x50): undefined reference to `vmstate_pci_device'

We must only compile ich.c if CONFIG_PCI is available, too, so introduce a
new config switch for this device.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>