error: Fix -msg timestamp default

-msg parameter "timestamp" defaults to "off" if you don't specify msg,
and to "on" if you do.  Messed up right in commit 5e2ac51917 "add
timestamp to error_report()".  Mostly harmless, because "timestamp" is
the only parameter, so "if you do" is "-msg ''", which nobody does.

Change the default to "off" no matter what.

While there, rename enable_timestamp_msg to error_with_timestamp, and
polish documentation.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191010081508.8978-1-armbru@redhat.com>

util/qemu-error.c

@@ -24,6 +24,9 @@ typedef enum {
     REPORT_TYPE_INFO,
 } report_type;
 
+/* Prepend timestamp to messages */
+bool error_with_timestamp;
+
 int error_printf(const char *fmt, ...)
 {
     va_list ap;
@@ -191,7 +194,6 @@ static void print_loc(void)
     }
 }
 
-bool enable_timestamp_msg;
 /*
  * Print a message to current monitor if we have one, else to stderr.
  * @report_type is the type of message: error, warning or informational.
@@ -204,7 +206,7 @@ static void vreport(report_type type, const char *fmt, va_list ap)
     GTimeVal tv;
     gchar *timestr;
 
-    if (enable_timestamp_msg && !cur_mon) {
+    if (error_with_timestamp && !cur_mon) {
         g_get_current_time(&tv);
         timestr = g_time_val_to_iso8601(&tv);
         error_printf("%s ", timestr);