tracing: Use double-dash spelling for trace option

The '-trace' and '--trace' spellings are only both supported in qemu
binary, while for qemu-nbd or qemu-img only '--trace' spelling is
supported. So for the consistency of trace option invocation, we
should use double-dash spelling in our documentation.

This's also mentioned in
https://wiki.qemu.org/BiteSizedTasks#Consistent_option_usage_in_documentation
.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1530674247-31200-1-git-send-email-baiyaowei@cmss.chinamobile.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/devel/tracing.txt

@@ -18,7 +18,7 @@ for debugging, profiling, and observing execution.
 
 3. Run the virtual machine to produce a trace file:
 
-    qemu -trace events=/tmp/events ... # your normal QEMU invocation
+    qemu --trace events=/tmp/events ... # your normal QEMU invocation
 
 4. Pretty-print the binary trace file:
 
@@ -157,11 +157,11 @@ The state of events can also be queried and modified through monitor commands:
 * trace-event NAME on|off
   Enable/disable a given trace event or a group of events (using wildcards).
 
-The "-trace events=<file>" command line argument can be used to enable the
+The "--trace events=<file>" command line argument can be used to enable the
 events listed in <file> from the very beginning of the program. This file must
 contain one event name per line.
 
-If a line in the "-trace events=<file>" file begins with a '-', the trace event
+If a line in the "--trace events=<file>" file begins with a '-', the trace event
 will be disabled instead of enabled.  This is useful when a wildcard was used
 to enable an entire family of events but one noisy event needs to be disabled.
 

trace/control.c

@@ -253,15 +253,15 @@ void trace_init_file(const char *file)
 #ifdef CONFIG_TRACE_SIMPLE
     st_set_trace_file(file);
 #elif defined CONFIG_TRACE_LOG
-    /* If both the simple and the log backends are enabled, "-trace file"
+    /* If both the simple and the log backends are enabled, "--trace file"
      * only applies to the simple backend; use "-D" for the log backend.
      */
     if (file) {
         qemu_set_log_filename(file, &error_fatal);
     }
 #else
     if (file) {
-        fprintf(stderr, "error: -trace file=...: "
+        fprintf(stderr, "error: --trace file=...: "
                 "option not supported by the selected tracing backends\n");
         exit(1);
     }

trace/control.h

@@ -193,7 +193,7 @@ void trace_event_set_vcpu_state_dynamic(CPUState *vcpu,
 /**
  * trace_init_backends:
  * @file:   Name of trace output file; may be NULL.
- *          Corresponds to commandline option "-trace file=...".
+ *          Corresponds to commandline option "--trace file=...".
  *
  * Initialize the tracing backend.
  *
@@ -204,7 +204,7 @@ bool trace_init_backends(void);
 /**
  * trace_init_file:
  * @file:   Name of trace output file; may be NULL.
- *          Corresponds to commandline option "-trace file=...".
+ *          Corresponds to commandline option "--trace file=...".
  *
  * Record the name of the output file for the tracing backend.
  * Exits if no selected backend does not support specifying the