Comment 18 for bug 1848497

The virtio_balloon_config size was changed in the Disco version of qemu because of bug 1836154, which backported the size change in patch:
ubuntu/lp-1836154-include-update-Linux-headers-to-4.21-rc1-5.0-rc1.patch

which was introduced in qemu version 1:3.1+dfsg-2ubuntu3.4:
http://launchpadlibrarian.net/438501259/qemu_1%3A3.1+dfsg-2ubuntu3.3_1%3A3.1+dfsg-2ubuntu3.4.diff.gz

so the fix from this bug also needs to be applied to the qemu in Stein