Move data from one device to another. For btrfs and lvm their specialized
utilities are used, so the data are moved in an all-or-nothing fashion and no
other operation is needed to add/remove the devices or rebalance the pool.
Devices that do not belong to a **backend** that supports specialized
device migration tools will be migrated using **dd**.

This operation is not intended to be used for duplication, because the process
can change metadata and an access to the data may be difficult.