I agree with you 鲨鱼兄。Guest is good

鲨鱼兄说："the poet used “guest” is totally subject to reads’ understanding and imagination."  

 

I think just because the poet didn't  use 臣虏 -- 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 作这个客非易事--要在梦里才忘记身为客这事实，可想而知醒时为客的负担.   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  臣虏。 So when it comes to get this line translated into English, I would use  a guest instead of a detained guest.