tools/libxl: store remus_ops in checkpoint device state

tools/libxl: store remus_ops in checkpoint device state

Checkpoint device is an abstract layer to do checkpoint.
COLO can also use it to do checkpoint. But there are
still some codes in checkpoint device which touch remus.

This patch and:
 tools/libxl: move remus state into a seperate structure
 tools/libxl: seperate device init/cleanup from checkpoint device layer
will seperate remus from checkpoint device layer.

We use remus ops directly in checkpoint device. Store it
in checkpoint device state so that we do not aware of
remus_ops in the checkpoint device layer.

It is pure refactoring and no functional changes.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Yang Hongyang <hongyang.yang@easystack.cn>
Acked-by:Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>