tools/libxl: move remus state into a seperate structure

tools/libxl: move remus state into a seperate structure

Add a new structure remus state, and move concrete layer's private
member to remus state.
it is pure refactoring and no functional changes.
Init interval in libxl__remus_setup(). It is safe to move this initialisation,
because this value is only used for remus, and remus will use this value after
libxl__remus_setup().

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