systemd with unified control group hierarchy

Systemd upstream now defaults to using unified control group hierarchy. In Ubuntu up to Hirsute this change is reverted in the systemd package to avoid regressions. Current plan is to not remove cgroups v1 before Ubuntu Impish (21.10). See bugs.launchpad.net/ubuntu/+source/snapd/+bug/1850667.

As most other distribution have already switched to unified control group hierarchy, Ubuntu containers that run systemd inside cannot be started anymore. A work-around is to manually mount systemd cgroup hierarchy on the host after boot before starting LXD:
sudo mount -t cgroup -o none,name=systemd systemd /sys/fs/cgroup/system

If snap is not required to run within the Ubuntu container systemd packages provided by this PPA can be used, as they go with upstream's default cgroup hierarchy.

Installation:
Host:
- Reboot with enabled cgroup hybrid mode by adding systemd.unified_cgroup_hierarchy=0 as kernel boot parameter
- Create directory for systemd cgroup v1 hierarchy:
   sudo mkdir -p /sys/fs/cgroup/system
   sudo mount -t cgroup -o none,name=systemd systemd /sys/fs/cgroup/system
- Restart LXD, then start Ubuntu Focal container and log in
Container:
- Install this PPA's systemd packages
- Optionally, remove lxd snap (for nested containers), which may be buggy
Host:
- Restart Ubuntu container

The Ubuntu container should then work on a unified cgroup host.