tools/kvm_stat: fix drilldown in events-by-guests mode

When displaying debugfs events listed by guests, an attempt to switch to
reporting of stats for individual child trace events results in garbled
output. Reason is that when toggling drilldown, the update of the stats
doesn't honor when events are displayed by guests, as indicated by
Tui._display_guests.
To reproduce, run 'kvm_stat -d' and press 'b' followed by 'x'.

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