trace: print a warning if user tries to enable an unknown trace event

There was no warning if a bad trace event name was given to
'trace-event' command, thus the user could think that the command
was successful even if this was not the case.

Print a warning if the user tries to enable a trace event which is not
known.

Acked-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>