libxl: introduce libxl_pci_bdf_assignable_add/remove/list/list_free(), ...

libxl: introduce libxl_pci_bdf_assignable_add/remove/list/list_free(), ...

which support naming and use 'libxl_pci_bdf' rather than 'libxl_device_pci',
as replacements for libxl_device_pci_assignable_add/remove/list/list_free().

libxl_pci_bdf_assignable_add() takes a 'name' parameter which is stored in
xenstore and facilitates two addtional functions added by this patch:
libxl_pci_bdf_assignable_name2bdf() and libxl_pci_bdf_assignable_bdf2name().
Currently there are no callers of these two functions. They will be added in
a subsequent patch.

libxl_device_pci_assignable_add/remove/list/list_free() are left in place
for compatibility but are re-implemented in terms of the newly introduced
functions.

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