This is to list the network filter objects. No flags are supported

include/libvirt/libvirt.h.in: Declare enum virConnectListAllNWFilterFlags
                              and virConnectListAllNWFilters.
python/generator.py: Skip auto-generating
src/driver.h: (virDrvConnectListAllNWFilters)
src/libvirt.c: Implement the public API
src/libvirt_public.syms: Export the symbol to public