tools/kvm_stat: fix crash when filtering out all non-child trace events

When we apply a filter that will only leave child trace events, we
receive a ZeroDivisionError when calculating the percentages.
In that case, provide percentages based on child events only.
To reproduce, run 'kvm_stat -f .*[\(].*'.

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