User volumes are identified by a short name which serves both
as a `/var/mnt` mount point and a partition label.
User volumes can be added and removed on the fly, and they are
automatically propagated into the `kubelet` mount namespace.
Also deprecate `.machine.disks`.
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>
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