tests: add tests for disk latency.

This test is similar in spirit to misc-bdev-write, but instead of pushing
as many bios as we can, we'll push one bio at a time.

Example output for Xen:

OSv v0.05-118-g0f2973c
Min      50%      90%      99%      99.99%   99.999%  Max     [msec]
---      ---      ---      ---      ------   -------  ---
0.2344   0.3240   0.2847   0.8185   2.4095   6.5230   13.6572

Example output fo KVM:

OSv v0.05-118-g0f2973c
Min      50%      90%      99%      99.99%   99.999%  Max     [msec]
---      ---      ---      ---      ------   -------  ---
0.2626   0.3976   0.3273   0.4791   0.5993   7.6401   15.9672

(Hint: the current xen blkfront slowness is not related to RT latency...)

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>