You will need get all or specific params use get function either way, so I don't think extract 'vol_' specific params for setting volume xml waste any much time then that.
And the benefit for do this is, many others params of volume xml are not added in my test cfg file, as they are not the focus test point here, but if others want add them they can simple add it to test cfg file without touch the test py file for adding specific params, even though they might still want some specific check methods for those params.
And why I not just give params dict to vol_xml module for simple solve all this, because some accessors in vol_xml require given value as integer, so I need to pick specific out and transfer them to int before pass in.