error: Report certain hints on stderr when no monitor

Hints printed with error_printf_unless_qmp() are suppressed outside
monitor context.  Reproducer:

    $ qemu-system-x86_64 -m 1Z
    qemu-system-x86_64: -m 1Z: Parameter 'size' expects a size

Print to stderr instead.  The reproducer now additionally prints:

    You may use k, M, G or T suffixes for kilobytes, megabytes, gigabytes and terabytes.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170105135957.12003-1-marcandre.lureau@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

monitor.c

@@ -3973,6 +3973,8 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
 {
     if (cur_mon && !monitor_cur_is_qmp()) {
         monitor_vprintf(cur_mon, fmt, ap);
+    } else if (!cur_mon) {
+        vfprintf(stderr, fmt, ap);
     }
 }