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>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index ad4174f..d77e75f 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -186,7 +186,7 @@
         .args_type  = "name:s,option:b",
         .params     = "name on|off",
         .help       = "changes status of a specific trace event",
-        .mhandler.cmd = do_change_trace_event_state,
+        .mhandler.cmd = do_trace_event_set_state,
     },
 
 STEXI