engine: Correct utc_diff handling

Now the utc_diff is always updated for running VMs (Linux or Windows)
and reset on VM run.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1062615
Change-Id: I907cd1ab724b6c3217cc90bd886b678d4bb9d37a
Signed-off-by: Martin Betak <mbetak@redhat.com>