To reproduce: try building some random package using a VM image produced by
autopkgtest-build-qemu, on a sid host.
I think the necessary change to copy from autopkgtest is this one:
* qemu: Guess format of main disk image (Closes: #968598)
from autopkgtest/5.14, but there have been a lot of other changes to
autopkgtest-virt-qemu, and it now depends on a new private module shared
with autopkgtest-build-qemu.
I would recommend depending on autopkgtest and running the autopkgtest
virt backends (autopkgtest-virt-*) from the PATH, rather than having
a private copy of them: the interface between autopkgtest and its virt
backends is stable (and documented!), even if the adtlog, adt_testbed
and VirtSubproc private Python modules are not currently suitable to be
shared as-is.
smcv