s390x qemu support in autopkgtest?

Dear s390x porters,

Recently I added isolation-machine [1] support to ci.debian.net on 
amd64. At this moment, I think performance wise the only architecture 
that qualifies in our infrastructure to add it too is s390x. However, 
autopkgtest-build-qemu doesn't know what to do yet. Would any of you 
have interest to improve autopkgtest-build-qemu to support building a 
qemu image on s390x?

It currently says this:
root@ci-worker-s390x-01:~# autopkgtest-build-qemu unstable unstable-s390x
Unable to guess an appropriate boot protocol, use --boot to specify

I note that ppc64el support was added in merge request 91 [2]. Obviously 
we'd also need to be able run these VM's via autopkgtest-virt-qemu, so 
if details are missing there.... (merge request related to ppc64el are 
linked from MR91).

Please note that this is the first time that I'm really working with 
qemu, so I'm learning as I go. My qemu experience is now several days.

Paul

[1] 
https://salsa.debian.org/ci-team/autopkgtest/-/blob/master/doc/README.package-tests.rst
[2] https://salsa.debian.org/ci-team/autopkgtest/-/merge_requests/91

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