This is a big deal. With some tooling around it can be amazing.
I can think of using this for testing, and as a vehicle to change a programming paradigm of existing/legacy software (run a thing, and roll it back aggressively from outside of a vm)
Indeed, the tooling is the problem. And I wouldn't hold my breath to see this tooling being implemented, as the feature has been around for quite a bit.
IMHO, PANDA [1] remains a better/more practical choice for whole-system record/replay analysis. It already offers quite a bit of tooling (including a python interface), as well as hooks to build your own. It does have its own shortcomings (speed and not being in-sync with the latest QEMU), but at least you're not limited to gdb-based debugging.
I can think of using this for testing, and as a vehicle to change a programming paradigm of existing/legacy software (run a thing, and roll it back aggressively from outside of a vm)