Support disk IO quota

Some or all of the instance quotas are currently not supported.

* disk_read_bytes_sec
* disk_read_bytes_sec

We also need quotas for Cinder volumes, see this excerpt from the libvirt code:

        # Extract rate_limit control parameters
        if 'qos_specs' in data and data['qos_specs']:
            tune_opts = ['total_bytes_sec', 'read_bytes_sec',
                         'write_bytes_sec', 'total_iops_sec',
                         'read_iops_sec', 'write_iops_sec']
            specs = data['qos_specs']