Add custom volume transfer to lxd-migrate

Currently lxd-migrate only supports moving full instances, but having support to move custom volumes too would be quite useful.

We should allow:

• Transfer a filesystem tree into a new filesystem type custom volume
• Transfer a block device into a new block type custom volume
• Allow for custom volumes to be added as part of migrating instances, so one can add a number of custom volumes as part of the migration and have those attached to the instance at the end of it