鯊魚兄說:"the poet used “guest” is totally subject to reads’ understanding and imagination."
I think just because the poet didn't use 臣虜 -- a detained guest, doesn't mean he was not. He didn't explain in details of what situation he was in, but he left readers the space to imagine and perceive on their own. At least reading the word 客, my understanding is that the poet 作這個客非易事--要在夢裏才忘記身為客這事實,可想而知醒時為客的負擔. On the other hand, I think that could be the respect and protection of his dignity as the former king of the nation made him use 客 instead of 臣虜。 So when it comes to get this line translated into English, I would use a guest instead of a detained guest.