tools/kvm_stat: add line for totals

Add a line for the total number of events and current average at the
bottom of the body.
Note that both values exclude child trace events. I.e. if drilldown is
activated via interactive command 'x', only the totals are accounted, or
we'd be counting these twice (see previous commit "tools/kvm_stat: fix
child trace events accounting").

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