qemu is no longer built on 32-bit architectures. Therefore, please
remove the Build-Depends: qemu-system from i386.

grub-firmware-qemu is built for any-amd64 any-i386. I assume that you
should stop
building it for any-i386.

You'll then need to file an architecture-specific removal bug to
enable grub2 to migrate to Testing.

I'm filing as serious because this issue is preventing the removal of
NBS binary packages from Unstable.

Thank you,
Jeremy Bícha