cgroup v2 support

[schu]

On systems with cgroup v2, kubedee (/lxd) prints the following warning on every invocation, so quite a lot:

WARNING: cgroup v2 is not fully supported yet, proceeding with partial confinement

Also, mounting devices might fails due to the devices cgroup not being mounted. Mounting the cgroup manually and restarting lxd after seems to work as a workaround:

mkdir /sys/fs/cgroup/devices
mount -t cgroup devices -o devices /sys/fs/cgroup/devices
snap restart lxd