This is a patch to do a job. Fairly simple one so I hope review won't be that long.
Things to consider:
1) Due to problem with "answers" field I didn't delete shortanswer code from xml format, so shortanswer question is still importing/exporting directly using format code right now. Hovewer my testing site run without "answers" field for almost a half-year. So I was able to test implementation on shortanswer question type - it is fully interoperable with an existing one in abscence of "answers" field. I did both exporting using new code while importing using old and vice versa. I also diffed xml files generated by existing and new export. All works OK.
Right now extended functionality will be available for any 3d party qtypes using question_extra_fields(). We can drop special handling of shortanswer from format.php after dropping "answers" field.
2) Performance of try_importing_using_qtypes() function was not great from the beginning as it loops throught all qtypes for each question and it will be even worse after implementing import_from_xml function in base questiontype class. I improved this by letting format to make a hint on qtype of the question. You may choose to use this part (changes in two format.php files) or not, it's up to you.