How to write a valid AAVMF file?

AAVMF files are not the same as OVMF 4M files it seems.

Setting "correct" filesize and blockmap does not appear to create valid AAVMF var files. See the naive approach in https://github.com/xnox/python-uefivars/commit/ba1919419b8e9a859ef5f1f5f515d2c3200e12b3

somehow with above patch doing:

```
uefivars -i edk2 -I /usr/share/AAVMF/AAVMF_VARS.ms.fd -o json -O ms.json
uefivars -i json -I ms.json -o edk2,preset=aavmf -O aavmf.ms.fd
truncate -s 64M aavmf.ms.fd
```

And booting the resulting firmware looses all variable settings, and the VM is no longer in secureboot mode.

Are some of the assumptions in writing edk2 format specific to OVMF and/or OVMF 4M size and/or blocksize?