list: Define new API virConnectListAllNodeDevices

list: Define new API virConnectListAllNodeDevices

This is to list the node device objects, supports to filter the results
by capability types.

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