It tests the accuracy of rtc periodic timer which is recently
improved & fixed by commit 7ffcb539a3 ("mc146818rtc: precisely count
the clock for periodic timer", 2017-05-19).

Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
Message-Id: <20170527025301.23499-1-xiaoguangrong@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>