libxl: introduce 'libxl_pci_bdf' in the idl...

libxl: introduce 'libxl_pci_bdf' in the idl...

... and use in 'libxl_device_pci'

This patch is preparatory work for restricting the type passed to functions
that only require BDF information, rather than passing a 'libxl_device_pci'
structure which is only partially filled. In this patch only the minimal
mechanical changes necessary to deal with the structural changes are made.
Subsequent patches will adjust the code to make better use of the new type.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wl@xen.org>
Acked-by: Nick Rosbrook <rosbrookn@ainfosec.com>