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>

include/qemu/error-report.h

@@ -73,6 +73,7 @@ void error_init(const char *argv0);
     })
 
 const char *error_get_progname(void);
-extern bool enable_timestamp_msg;
+
+extern bool error_with_timestamp;
 
 #endif