provider/maas: volume source won't work for physical machines/disks

The MAAS volume source rightly uses the HardwareId field of storage.Volume to correlate with block devices, for physical machines/disks (DeviceName is used for virtio disks). However, the work to populate BlockDevice.HardwareId in the diskmanager worker was never done, so it won't work properly. Block storage will work (I think?) but volume-backed-filesystems definitely will not.