Add context around QEMU flags steps

At the time of writing https://developer.puri.sm/Apps/GNOME/GBuilder.html#setting-up-qemu-on-debian-based-systems, this fix had not been upstreamed yet to be in PureOS. Now this is no longer needed on PureOS or Debian buster (although it is still needed for Debian stable systems). So rather than lose this information, some context should be added around this.