scripts/kvm/kvm_stat: Fixup filtering

When filtering, the group leader event should not be disabled, as all
other events under it will also be disabled. Also we should make sure
that values from disabled fields will not be displayed.

This also filters the fields from the log and batch output for better
readability.

Also the drilldown update now directly checks for the stats' field
filter and (un)sets drilldown accordingly.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-33-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
