References
http://www.nodalink.com/blog_throttling_25_01_2014.html
[[,bps=b]|[[,bps_rd=r][,bps_wr=w]]][[,iops=i]|[[,iops_rd=r][,iops_wr=w]]
[--domain] <string> domain name, id or uuid
[--device] <string> block device
--bps <number> total throughput limits in bytes/s
--bps_rd <number> read throughput limits in bytes/s
--bps_wr <number> write throughput limits in bytes/s
--iops <number> total operation limits in numbers/s
--iops_rd <number> read operation limits in numbers/s
--iops_wr <number> write operation limits in numbers/s
