trace: separate trace event control and query routines from the simple backend

Generalize the 'st_print_trace_events' and 'st_change_trace_event_state' into
backend-specific 'trace_print_events' and 'trace_event_set_state' (respectively)
in the "trace/control.h" file.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>

trace/default.c

diff --git a/trace/default.c b/trace/default.c
index 42fdb6b..3573d5b 100644
--- a/trace/default.c
+++ b/trace/default.c
@@ -10,6 +10,21 @@
 #include "trace/control.h"
 
 
+void trace_print_events(FILE *stream, fprintf_function stream_printf)
+{
+    fprintf(stderr, "warning: "
+            "cannot print the trace events with the current backend\n");
+    stream_printf(stream, "error: "
+                  "operation not supported with the current backend\n");
+}
+
+bool trace_event_set_state(const char *name, bool state)
+{
+    fprintf(stderr, "warning: "
+            "cannot set the state of a trace event with the current backend\n");
+    return false;
+}
+
 bool trace_backend_init(const char *file)
 {
     if (file) {