autopkgtest-build-qemu: build VM images with vmdb2

Antonio Terceirorequested to merge
terceiro/autopkgtest:build-qemu into master

Ubuntu images can't be created ATM due to a bug in Ubuntu's grub2 package: https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1786699

However, if I locally hack vmdb2 to not pass the command line option removed by Ubuntu, Ubuntu images can be created just fine.

Both Debian and Ubuntu images created have been tested with autopkgtest.

Closes: #904972

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