migration: fix moving containers with storage api
migration: fix moving containers with storage api

- copying between a non-storage-api and a storage-api LXD instance:
  - Detect a pool by looking at the containers profiles on the remote instance.
    If no root disk device with an appropriate pool is found, look if only a
    single pool exists and if so, use it. Otherwise fail.
- copying between a storage-api and a storage-api LXD instance
  - We currently do not allow to target a specific storage pool on the remote so
    for now check whether the pool exists, if not perform the same algorithm as
    in the non-storage-api to storage-api LXD instance case.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

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