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>
diff --git a/qemu-options.hx b/qemu-options.hx
index c63e794..b2cb1de 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4167,13 +4167,17 @@
 
 DEF("msg", HAS_ARG, QEMU_OPTION_msg,
     "-msg timestamp[=on|off]\n"
-    "                change the format of messages\n"
-    "                on|off controls leading timestamps (default:on)\n",
+    "                control error message format\n"
+    "                timestamp=on enables timestamps (default: off)\n",
     QEMU_ARCH_ALL)
 STEXI
 @item -msg timestamp[=on|off]
 @findex -msg
-prepend a timestamp to each log message.(default:on)
+Control error message format.
+@table @option
+@item timestamp=on|off
+Prefix messages with a timestamp.  Default is off.
+@end table
 ETEXI
 
 DEF("dump-vmstate", HAS_ARG, QEMU_OPTION_dump_vmstate,