lib/pci: generalize pci_cap_walk

Increase the utility of pci_cap_walk by allowing the caller to supply
the table of cap handlers via a single handler, which implements all
the cases of interest.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>