• vmmapi.h: Include MD header at the top of the file
• amd64/vmmapi_amd64.h: Add header guards
• Makefile: Use MACHINE for the MD directory name
• Add vmmapi_internal.{c, h} for internal MI functions
• Add amd64 internal header with struct vmctx definition

One more question, did you test it with AMD and Intel CPUs?
I can help to test with Intel, unfortunately my Ryzen is gone.

Tested using the following guests:

The only test I did for the patch was to compile the source code. When the patch code is stabilized I will try to do a proper test by running a virtual machine with bhyve.

I've rewritten the patch to make the diff readable.