tools/libxl: add back channel support to write stream

tools/libxl: add back channel support to write stream

Add back channel support to write stream. If the write stream is
a back channel stream, this means the write stream is used by
Secondary to send some records back.

Note: The function libxl__stream_write_checkpoint_state() will be used
in later patches called "secondary vm suspend/resume/checkpoint code" and
"primary vm suspend/resume/checkpoint code".

Signed-off-by: Yang Hongyang <hongyang.yang@easystack.cn>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>