The disk size of a diskless instance is 0, not None

For diskless instances it is still reasonable to sum up the disk usage
of all the (zero) disks, resulting in the empty sum. This uniformity
also has the advantage that iallocators (like hail) do not have to do
any special considerations for diskless instances.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>