I added exactly one attribute to the -listxml output that's necessary for a front-end to properly deal with slot cards that enable software lists (e.g. Acorn "tube" devices, Spectrum DOS expansions, WafaDrive, etc.), and provided a reference implementation for showing available software lists for a given slot configuration, and discovering a suitable configuration to run a given piece of software. It's very rare that I change the -listxml at all, and I try to avoid changing existing parts because it can break front-ends. Unfortunately qmc2 seems to be upset by the unrecognised attribute I added.

If you want to keep using qmc2, you need to update or keep using an old version of MAME with it. If you don't update it, you'll still be missing out on support for software lists enabled by slot cards. Prior to 0.217, MAME was not providing the necessary information, and using it effectively will require UI work.