kvm tools: Add IO delay option

Add a command line debug option to add a fix amount of delay in read and
write operation.

From Ingo "the delays are *constant* [make sure you use a high-res timers
kernel], so they do not result in nearly as much measurement noise as real
block IO does.
[...]
This way you are basically 'emulating' a real disk drive but you will
emulate uniform latencies, which makes measurements a lot more
reliable - while still relevant to the end result."

Suggested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Prasad Joshi <prasadjoshi124@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>