Is it possible to withdraw a conference paper after the rebuttal period?
I suggest that you write to the chair of the program committee, explaining that (1) you agree with the reviewers about what was missing from your paper, (2) you've revised the paper to correct the problem, (3) you realize (or at least believe) that it's too late to modify your submission to this conference, (4) you would therefore like to submit the the revised version elsewhere, and (5) you would therefore like to withdraw your submission to this conference.  I would expect a reasonable program chair to let you withdraw the paper; after all, the conference gains nothing by forcing you to wait for an official rejection.  

I'd also suggest mentioning that you're grateful to the reviewers for pointing out the problem.