libxl: add libxl_device_pci_assignable_list_free()...

libxl: add libxl_device_pci_assignable_list_free()...

... to be used by callers of libxl_device_pci_assignable_list().

Currently there is no API for callers of libxl_device_pci_assignable_list()
to free the list. The xl function pciassignable_list() calls
libxl_device_pci_dispose() on each element of the returned list, but
libxl_pci_assignable() in libxl_pci.c does not. Neither does the implementation
of libxl_device_pci_assignable_list() call libxl_device_pci_init().

This patch adds the new API function, makes sure it is used everywhere and
also modifies libxl_device_pci_assignable_list() to initialize list
entries rather than just zeroing them.

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