Change the UST event hash table match function

The event are now matched, when looked up, with the name/filter/loglevel
triplet since the UST tracer now allows us to enable the same event with
different filters or/and loglevels.

The disable command however only takes the event name so for now it
disable all events matching that name which is why we still hash by
event name.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>