Diskmanage: also include partitions with get_disks if flag is set

Diskmanage: also include partitions with get_disks if flag is set

and have a parent key for partitions, to be able to see the associated disk in
the result without having to rely on naming heuristics (just adding a number at
the end doesn't work for NVMes).

The disk's usage will not be based on the partitions usage if the flag is set,
but will simply be 'partitions'.

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