gfxinfo/amdgpu: fix the base_name generation + more tweaks + 100% code coverage \o/

Really needed to have unit tests for amdgpu to catch this silly regression... so I added tests for it and all the other databases :)

Since I was already breaking the backwards compatibility, I decided to break it a little more and try to align AMDGPU with Intel GPUs a little more. And since was at it, I also did the same with the VirtIO GPUs.

Fixes: 79b99b51 ("gfxinfo/amdgpu: make the gfx version an integer")