`XDPBenchMeasurements` results used to use `FlowMeasurementResults`
as a base class overriding some of its methods because xdp-bench
tool doesn't measure CPU usage and so `FlowMeasurementResults`
CPU metrics were set to `None`.

Recent changes from [0] now expects `FlowMeasurementResults` to
use both perf and CPU results. Especially
`.{start,end}_timestamp` properties expect CPU metrics to be set
and if not, it crashes since not using CPU metrics is "masked"
by setting `None` instead of regular result containers.

[0] LNST-project#382

`JsonRunSummaryFormatter` requires measurement results to be
instance of `MeasurementResult` class.