api: backup: require Datastore.Allocate on storage

api: backup: require Datastore.Allocate on storage

In particular this ensures that the user is allowed to remove data on
the storage, because configuring low retention results in removed
older backups. Of course setting the storage itself also needs to
require the same privilege then.

This is a breaking API change, but it seems sensible to require
permissions on the affected storage too.

Jobs with a dumpdir setting can be configured by root only.

Suggested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>