Comment 31 for bug 1913421

Iteration I:
- Bionic's and Focal's systemd do not know "ReadWriteOnly".
  That is not super-important to have, dropped from those backports
  (done)
- default mount options slightly differ >=Hirsute vs <=Focal, but nothing
  that affects the use-case
  (no action needed)
- Removing qemu-block-extra >=Hirsute has dependency issues due to -gui
  I verified that is not due to anything we change hereby
  (no action needed)
- F/B stopped the mount unit (but we want to keep it for qemu's that are around)
  (needs debugging)
- F/B had no modules saved on --reinstall
  (needs debugging)

Added test step after each action:
- also check the set of saved modules present after every action

TODO before iteration II:
 - F/B stopped the mount unit
 - F/B had no modules saved on --reinstall