trace: Replace error with warning if event is not defined At the moment QEMU exits if trace point is not defined which makes a developer life harder if he has to switch between branches with different traces implemented. This replaces error+exit wit WARNING if the tracepoint does not exist or not traceable. Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>