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: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>