add diskio throttling option to drive

add diskio throttling option to drive

This add disk io limit to drive options.

I also add the qemu monitor command, but I din't have added yet to Qemu.pm

>From qemu mailing:

Some available features follow as below:
(1) global bps limit.
   -drive bps=xxx            in bytes/s
(2) only read bps limit
   -drive bps_rd=xxx         in bytes/s
(3) only write bps limit
   -drive bps_wr=xxx         in bytes/s
(4) global iops limit
   -drive iops=xxx           in ios/s
(5) only read iops limit
   -drive iops_rd=xxx        in ios/s
(6) only write iops limit
   -drive iops_wr=xxx        in ios/s
(7) the combination of some limits.
   -drive bps=xxx,iops=xxx

Known Limitations:
(1) #1 can not coexist with #2, #3
(2) #4 can not coexist with #5, #6
(3) When bps/iops limits are specified to a small value such as 511 bytes/s,
this VM will hang up. We are considering how to handle this senario.

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>