memory.c: Use trace_event_get_state_backends()

memory.c: Use trace_event_get_state_backends()

The preferred way to test whether a trace event is enabled is to
use trace_event_get_state_backends(), because this will give the
correct answer (allowing expensive computations to be skipped)
whether the trace event is compile-time or run-time disabled.
Convert the four old-style direct uses of TRACE_FOO_ENABLED in
memory.c.

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