`limits.cpu.nodes: balanced` can cause invalid config value for `volatile.cpu.nodes`

[gibmat]

Required information

• Distribution: Debian
• Distribution version: bookworm
• Incus version 1:6.7-debian12-202411151750 from zabbly

Issue description

When limits.cpu.nodes: balanced is set in a config, Incus can set the value of volatile.cpu.nodes to "0" when creating an instance. However, this triggers a validation error when later attempting to edit an instance's config. Might be related to 676ba92cfceff9c71b3dee70eb983a14540e890f; I don't remember seeing this prior to the Incus 6.7 release.

Steps to reproduce

1. Add limits.cpu.nodes: balanced to a profile:

   $ incus profile show default
   config:
   limits.cpu.nodes: balanced
   description: Default Incus profile
   devices:
   eth0:
   name: eth0
   network: incusbr0
   type: nic
   root:
   path: /
   pool: default
   type: disk
   name: default

2. Create and start a new VM:

   $ incus create --vm images:ubuntu/24.04
   Creating the instance
   Instance name is: dynamic-yak
   $ incus start dynamic-yak

3. View the config for this pristine VM:

   $ incus config show dynamic-yak
   architecture: x86_64
   config:
   image.architecture: amd64
   image.description: Ubuntu noble amd64 (20241126_07:42)
   image.os: Ubuntu
   image.release: noble
   image.requirements.cgroup: v2
   image.serial: "20241126_07:42"
   image.type: disk-kvm.img
   image.variant: default
   volatile.base_image: 09fd1be534aed9b11b5c3a8306619b13a542acbd74d376570e51774f90b405f1
   volatile.cloud-init.instance-id: dfdd1d5a-e0f2-4b8e-8774-7c7ddc3df08c
   volatile.cpu.nodes: "0"
   volatile.eth0.host_name: tap5d9590ac
   volatile.eth0.hwaddr: 00:16:3e:b1:91:d5
   volatile.last_state.power: RUNNING
   volatile.uuid: fd0d1c15-b074-4883-877c-f9289842cecc
   volatile.uuid.generation: fd0d1c15-b074-4883-877c-f9289842cecc
   volatile.vsock_id: "917373356"
   devices: {}
   ephemeral: false
   profiles:
   - default
   stateful: false
   description: ""

4. Open the config in an editor, and close without making any changes:

   $ incus config edit dynamic-yak
   Config parsing error: Invalid config: Invalid cpuset value: 0
   Press enter to open the editor again or ctrl+c to abort change
   ^C

[stgraber]

That's already been fixed :)