OK, someone on the Vbox forum gave me the solution. You can indeed edit the number of CPUs allocated, but only after having shut down the guest completely. There are three ways to quit a guest:
1. Save the machine state
2. Send the shutdown signal
3. Power off the machine
Normally, in this test at least, you would always save the state, so that you can directly boot into whatever you were doing. But in order to edit the # of CPUs, you have to choose "Power off the machine".
After reboot (of the guest), FAH checked my line in config.xml and continued the WU with 24 CPUs as if nothing happened.
EDIT: A note on the go. Vbox handles the cores/threads in a somewhat strange manner, at least as seen through its interface. The processor here has 14C/28T. In the settings, I can allocate up to 28 "CPUs" to the guest (14 in the green zone, 14 more in the orange zone), but then it generates an error, well sorta. I allocated 24 things and the system gives me a message telling that I allocated more CPUs than I actually have and that this might affect the performance of the guest. But those really are threads, that is, if I allocated only 12 of those "CPUs", I only have 12 at my disposal in FAHControl. So that's that. And the fact is that the performance is disappointing: I can already say that there's obviously no point using VBox and Ubuntu (on a Win 10 setup) in order to optimize the output of the hardware for FAH.
EDIT2: Maybe the guest doesn't have enough juice. I tried to give it 25 threads (lighter color on the table). The difference is not evident but there is something and looking at a top in Ubuntu, it seems that FAH cannot fully use the processor. I'll stop the folding on the GPUs and allocate all 28 threads to the Ubuntu guest to see if that helps. Results in a lighter color yet.
EDIT3: It appears that Vbox needs some extra "free" CPUs in order for FAH to deliver. But it is still way beyond LN and WSL2. I stop the Vbox test here and I'll look into VSphere, hopefully with GPUs, next week-end.
EDIT4: The next test will be late: my main machine is dead. Either the CPU of the MB. It's in RMA.
EDIT5: AMD replaced the 3970x, but I've become concerned about using it for FAH. Too many errors 997. It might change if AMD can ascertain the origin of the failure, or if FAHControl stops producing those errors. As for this test, I lack motivation. Obviously, if you want efficiency you have to use Linux and that's not what I want, apart maybe with WSL2. So it's on ice.
