Device override should fallback to device set

[simondeziel]

This works:

$ lxc init v1 --empty --vm
$ lxc config device override v1 root size=50GiB

But when a specific storage pool is specified during init, the device override no longer works, device set needs to be used instead:

$ lxc init v1 --empty --vm -s butter  # alternate storage pool
$ lxc config device override v1 root size=50GiB
Error: The device already exists

While that makes sense from an instance config point of view:

$ lxc config show v1
architecture: x86_64
config:
  volatile.apply_template: create
  volatile.cloud-init.instance-id: e51b383f-1f65-401a-9c2d-669cc77ecb8d
  volatile.eth0.hwaddr: 00:16:3e:f9:0c:a6
  volatile.uuid: 01dbcee9-1a26-4b01-8cea-c4d0c590a4c0
  volatile.uuid.generation: 01dbcee9-1a26-4b01-8cea-c4d0c590a4c0
devices:
  root:
    path: /
    pool: butter
    type: disk
ephemeral: false
profiles:
- default
stateful: false
description: ""

The workaround is to use device set (lxc config device set v1 root size=50GiB) but the user experience could be improved by allowing device override fallback to being a device set.