failed to start instance (Missing source '/dev/null' for disk 'aadisable')

[falfaro]

When deploying the "canonical-kubernetes" spell on an Ubuntu 17.04 system and LXC 2.1, Juju machines get in failed state with the following error:

Machine  State    DNS  Inst id  Series  AZ  Message
0        down          pending  xenial      failed to start instance (Missing source '/dev/null' for disk 'aadisable'), retrying in 10s (3 more attempts)

The only workaround I found, which I'm not sure if it yields a working k8s cluster or not, consists of changing the lxd-profile.yaml file and removing the entire "devices" section (for aadisable and aadisable1).