libxl: stop setting 'vdevfn' in pci_struct_fill()

libxl: stop setting 'vdevfn' in pci_struct_fill()

There are only two call-sites. One always sets it to 0 (which is unnecessary
as the structure is already initialized to zero) and the other can simply set
the 'vdevfn' field directly (after proper structure initialization), avoiding
the need for a local variable.

A subsequent patch will also make use of pci_struct_fill() in a context
where 'vdevfn' may already have been set.

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