Comment 1 for bug 1866032

These packages are only Build-Depends-Indep, meaning they are only needed when you are building the arch: all packages from the qemu source package.

The following command succeeds on s390x with an up-to-date focal system:

# apt build-dep --arch-only qemu

If you need to rebuild qemu-system-data, the only arch: all binary package from the qemu source, you will want to do that rebuild on an x86 system. Official arch: all builds for the Ubuntu archive are always done on amd64, so Build-Depends-Indep are not guaranteed to be satisfiable on other architectures.