x86: don't pass negative time to gtime_to_gtsc() (try 2)

x86: don't pass negative time to gtime_to_gtsc() (try 2)

This mostly reverts commit eb60be3d ("x86: don't pass negative time to
gtime_to_gtsc()") and instead corrects __update_vcpu_system_time()'s
handling of this_cpu(cpu_time).stime_local_stamp dating back before the
start of a HVM guest (which would otherwise lead to a negative value
getting passed to gtime_to_gtsc(), causing scale_delta() to produce
meaningless output).

Flushing the value to zero was wrong, and printing a message for
something that can validly happen wasn't very useful either.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>