Verify overall workers ratio is as expected

[ldoktor]

In case we have multiple workers, they should perform in a certain ratio (equal in case of symmetric configuration). Let's add a check for that which means yielding another result per each test result:

DefaultLibvirt/fio/0000:./read-4KiB/throughput/iops_sec.mean_ratio: 3

(means that the average ratio was off by 3%; using absolute values to avoid masking values; eg. 25:25:25:25 ratio where real ratio is 24.25:25.75:25:25 would result in -3%:+3%:0%:0%, therefore (3+3+0+0)/4 = 1.5)

Later we might even include quality of mean_ratio that would signal how frequently is the ratio broken during the test execution (for tests that are sampling throughout the test execution):

DefaultLibvirt/fio/0000:./read-4KiB/throughput/iops_sec.mean_ratio_samples: 13

Similar to mean_ratio only calculated per each sample (at time 0, 1, 2, 3...) throughout the test execution.