do not add unused disk when already as unused on another storage

do not add unused disk when already as unused on another storage

when having an unused disk on a storage for which there are multiple
definitions, we added it again on another storage when that storage
was alphabetically before the already existing one

this happens for example when using our automatically generated
ceph storages: 'pool_ct' and 'pool_vm' and having a vm with
an unused disk

with this patch, we also leave the unused disks in the hash

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>