docs/devel/tracing.txt: Recommend only trace_event_get_state_backends()

docs/devel/tracing.txt: Recommend only trace_event_get_state_backends()

Instead of recommending checking the TRACE_FOO_ENABLED macro to
skip expensive computations needed only for tracing, recommend
only using trace_event_get_state_backends(). This works for both
compile-time and run-time disabling of events, and has no extra
performance impact if the event is compile-time disabled.

Signed-off-by: Peter Maydell <[email protected]>
Message-id: 20200120151142[email protected]
Message-Id: <20200120151142[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>