tools/kvm_stat: fix child trace events accounting

Child trace events were included in calculation of the overall total,
which is used for calculation of the percentages of the '%Total' column.
However, the parent trace envents' stats summarize the child trace
events, hence we'd incorrectly account for them twice, leading to
slightly wrong stats.
With this fix, we use the correct total. Consequently, the sum of the
child trace events' '%Total' column values is identical to the
respective value of the respective parent event. However, this also
means that the sum of the '%Total' column values will aggregate to more
than 100 percent.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>