Add support for additional disks to incus-migrate

[stgraber]

Is there an existing issue for this?

• There is no existing issue for this feature

What are you currently unable to do

As a follow-up to #1877, we should then allow adding additional disks to a container or VM as part of their migration. When doing so, those extra disks would first be transferred as custom volumes before the instance itself is transferred and the disks connected.

For containers, we'll only allow extra file system custom volumes (path).
For VMs, we'll support either file system or block (including qcow2 and vmdk as supported for the root volume).

What do you think would need to be added

No response