Cleanup: tracepoint event: use different prefixes for provider and event descriptors

Prefix the provider array of descriptor identifier with
"lttng_ust__provider_event_desc___" to distinguish it from the event
descriptor identifier.

This does not cause any conflict even when the event has the same name
as the provider because the provider is included in the event name. But
providing different prefixes is cleaner.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: If8b38d2765de61fd59558fe7c6223d1fdbb3349b