tools/libxl: introduce enum type libxl_checkpointed_stream

tools/libxl: introduce enum type libxl_checkpointed_stream

Introduce enum type libxl_checkpointed_stream in IDL.
rename the last argument of migrate_receive from "remus" to
"checkpointed" since the semantics of this parameter has
changed.

NOTE:
 libxl_domain_restore_params and domain_create aren't changed here,
 checkpointed_stream is still an int. Because we will pass the
 value from libxl to libxc.

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