As vlib_buffer_clone/copy() function preserve packet trace bit in its flags field, it should also copy trace_handle() as default action.  This can prevent uninitialized trace_handle in the new buffer be used by packet trace handling and also relieve callers from having to call vlib_buffer_copy_trace_flags() to copy trace_handle().  Caller of vlib_buffer_clone/copy() function always have the option to call vlib_trace_buffer() to get a new trace_handle for the new buffer so its packet trace is separated from the original packet.