tools/kvm_stat: group child events indented after parent

We keep the current logic that sorts all events (parent and child), but
re-shuffle the events afterwards, grouping the children after the
respective parent. Note that the percentage column for child events
gives the percentage of the parent's total.
Since we rework the logic anyway, we modify the total average
calculation to use the raw numbers instead of the (rounded) averages.
Note that this can result in differing numbers (between total average
and the sum of the individual averages) due to rounding errors.

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