disk api: only ask for Datastore.Allocate if adding to storage config

disk api: only ask for Datastore.Allocate if adding to storage config

The Proxmox VE storage systems doesn't cares at all if the
Datastore.Allocate privilege is present if no Proxmox VE storage will
be allocated.

Note, if we want to restrict this further as Sys.Modify on /, which
is already quite a powerful permission, we should probably add a new
one under the Sys. space, e.g., Sys.Disk.Use or the like.

This is a step in splitting the disk manage code out of the
pve-storage package, and maybe even repository

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>