Disk utilisation plugin.

Returns metrics for all devices listed by iostat that do not start with
'dm-'. The metric is the %util over a period of 5 seconds from when the
check is run.

Required by the CDM checks/alarms patch in ansible-lxc-rpc
I've addressed this in the above comment I think but, I'm not sure this really helps because there shouldn't be empty lists and if there is something has changed.