tools/libxl: move remus code into libxl_remus.c

tools/libxl: move remus code into libxl_remus.c

After previous refactoring, we are now able to move all remus code
into a separate file libxl_remus.c.

Export following functions for internal use:
- setup/teardown Remus:
  * libxl__remus_setup
  * libxl__remus_teardown
  * libxl__remus_restore_setup

Signed-off-by: Yang Hongyang <hongyang.yang@easystack.cn>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
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>