libxl/remus: init checkpoint callback in Remus setup callback

libxl/remus: init checkpoint callback in Remus setup callback

Init stream {read/write} state checkpoint_callback, suspend/resume/checkpoint
callback in Remus setup callback.
There's no functional change, it's just refactoring so that we can move
all remus code into one file.

Signed-off-by: Yang Hongyang <hongyang.yang@easystack.cn>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>