list: Define new API virConnectListAllSecrets

list: Define new API virConnectListAllSecrets

This is to list the secret objects. Supports to filter the secrets
by its storage location, and whether it's private or not.

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