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

tools/kvm/kvm_stat/kvm_stat

@@ -1084,9 +1084,15 @@ class Tui(object):
         self.screen.clrtobot()
         stats = self.stats.get(self._display_guests)
         total = 0.
+        ctotal = 0.
         for key, values in stats.items():
             if key.find('(') == -1:
                 total += values.value
+            else:
+                ctotal += values.value
+        if total == 0.:
+            # we don't have any fields, or all non-child events are filtered
+            total = ctotal
 
         if self._sorting == SORT_DEFAULT:
             def sortkey((_k, v)):