Add support for a duration/length for the bps/iops and friends.
Modify the API in order to add the "blkdeviotune." specific definitions
for the iotune throttling duration/length options
total_bytes_sec_max_length
write_bytes_sec_max_length
read_bytes_sec_max_length
total_iops_sec_max_length
write_iops_sec_max_length
read_iops_sec_max_length