Still when "unlisted datastore can be used for all purposes"
The ZVM does not allow the disks to be used for pre-seed (volume are not listed on the ZVM)
From the ZVM on the Site settings:
Accessed from Site Settings > Cloud Settings tab > Datastore Configuration area
In the Provider vDCs area (top), click Add.
And Configure Provider > "Provider Datastore" you will specifically need to select the "pre-seed" possibility to that datastore from the available options: [mirror], [journal] and [pre-seed].