api: move-volume: add move to another container

api: move-volume: add move to another container

The goal of this is to expand the move-volume API endpoint to make it
possible to move a container volume / mountpoint to another container.

For unused volumes, the API parameters have been changed to allow them
as well. This means, additional checks had to be introduced to avoid
migration of an unusedX volume to another storage. Some follow up work
is needed for that to work properly.

Moving the rootfs from or to another container is prohibited.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>