Diskmanage: introduce usage helper

Diskmanage: introduce usage helper

Note that this is a slight behavior change, because now the first
partition's usage which is not simply 'partition' will become the disk's
usage. Previously, if any partition was 'mounted', it would become the disk's
usage, then 'LVM', 'ZFS', etc.

A partitions usage defaults to 'partition' if nothing more specific can be
found, and is never treated as unused for now.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>