libxl: stop using aodev->device_config in libxl__device_pci_add()...

libxl: stop using aodev->device_config in libxl__device_pci_add()...

... to hold a pointer to the device.

There is already a 'pci' field in 'pci_add_state' so simply use that from
the start. This also allows the 'pci' (#3) argument to be dropped from
do_pci_add().

NOTE: This patch also changes the type of the 'pci_domid' field in
      'pci_add_state' from 'int' to 'libxl_domid' which is more appropriate
      given what the field is used for.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Wei Liu <wl@xen.org>