B450M Steel Legend can't boot

The AGESA is basically a firmware roadmap for all the functionality integrated
into the CPU. A while back now all motherboards contained a northbridge and
southbridge chipset, these controlled the vast majority of board functions and
put control over features predominantly in the hands of board manufacturers, at
least to a much larger extent than today. Now, the northbridge, or it's equivalent,
is integrated into the CPU (both intel and AMD).

This is one of the key reasons boards get blamed for problems more now than ever
before. Add to that the fact that the memory controller is likewise now integrated
into the CPU and you start seeing a pattern.
As for VMs, the AGESA controls just about every feature that would have been on
the northbridge, including critical functionality needed for VMs. Given how much
of a niche VM users are, AMD will often overlook bugs and glitches in this portion
of their code, choosing instead to focus on more broadly relevant issues. That
is why certain AGESA versions are better and worse for things like VMs and other
less commonly used features.