Simply returns the storage volume objects. No supported filter
flags.

include/libvirt/libvirt.h.in: Declare the API
python/generator.py: Skip the function for generating. virStoragePool.py
                     will be added in later patch.
src/driver.h: virDrvStoragePoolListVolumesFlags
src/libvirt.c: Implementation for the API.
src/libvirt_public.syms: Export the symbol to public