tools/libxl: seperate device init/cleanup from checkpoint device layer

tools/libxl: seperate device init/cleanup from checkpoint device layer

we call (init|cleanup)_subkind_nic and (init|cleanup)_subkind_drbd_disk
directly in checkpoint device. Move them to libxl_remus.c, Call them before
calling libxl__checkpoint_devices_setup() or after calling
libxl__checkpoint_devices_teardown().
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>