storage: Fix a potential crash when creating vol object

storage: Fix a potential crash when creating vol object

If the vol object is newly created, it increases the volumes count,
but doesn't decrease the volumes count when do cleanup. It can
cause libvirtd to crash when one trying to free the volume objects
like:
    for (i = 0; i < pool->volumes.count; i++)
        virStorageVolDefFree(pool->volumes.objs[i]);

It's more reliable if we add the newly created vol object in the
end.