Otherwise it fails with:
umount: /tmp/mmdebstrap.Tw9G7ZLL4J/mnt: filesystem was unmounted, but failed to update userspace mount table.
E: setup failed: E: command failed: umount --lazy "$1/mnt"
Also umount mnt/dev.
- explicitly instruct to add --boot=efi to autopkgtest-virt-qemu
- add example how to run autopkgtest with --boot=efi
- document image location requirements giving unshare restrictions
- check if foreign arch is configured
- instruct how to add a foreign architecture
- check that the unshared user is able to access the image location
- suggest to install qemu-system-* packages if they are missing
- suggest to install packages containing EDK II OVMF UEFI firmware