Steal time constantly goes down

[xemul]

The formula is

    return std::chrono::duration_cast<std::chrono::nanoseconds>(now() - _start_time - _total_sleep) -
           std::chrono::duration_cast<std::chrono::nanoseconds>(thread_cputime_clock::now().time_since_epoch());

it does (time_point - time_point - duration) - time_point which it technically "time", but substracting time_point from duration shouldn't work

[xemul]

[travisdowns]

I have a ~fox~ fix for this.

https://github.com/scylladb/seastar/pull/2390