Rebase isotesters' custom QEMU on 1:2.8+dfsg-4

Originally created by @intrigeri on #12468 (Redmine)

We currently run 1:2.8+dfsg-3.0tails1 and a newer package was uploaded to Debian and will make it into testing shortly.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information