This avoids taking the active_timers_lock or resetting/setting the
timers_done_ev if there are no active timers.  This removes a small
(2-3%) source of overhead for dataplane.  The list is then checked
again inside the lock, or a NULL pointer could be dereferenced.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>