This may betray my own ignorance, but I believe 中国人 is perfectly fine for describing oneself even if you are not from the Mainland, such as in my case. I've hung around plenty of 1st generation Chinese folk from Taiwan and conversation seems to still use 中国人 as a general term, not as something specific only to Mainlanders. I suppose if you really wanted, you could use the term "Han Chinese", but that is in fact more ethnically specific than 中国人.