sbuild-qemu fixes

@ckk do you think you could find some time to clean up this MR? Feel free to force-push to this branch. This is just a proof-of-concept for now and fixes the issues found by Francesco Poli in #1061636. The most important issue is, that mmdebstrap-autopkgtest-build-qemu creates images using efi, so sbuild-update-qemu needs a way to use efi booting independent on the architecture. So I added a --boot option. While working on that I added some other quality-of-life changes like a --verbose option which is useful to figure out what is wrong when expect doesn't find what it is supposed to and the ability to pass any arbitrary arguments to qemu which is useful for things that will never land in sbuild-qemu but are useful locally. Before this can be merged, this should:

• also be applied to sbuild-qemu-boot possibly doing some refactoring so that code can be shared
• handle errors
• add documentation in --help as well as the man pages

Thanks!