qtest: Add missing GCC_FMT_ATTR

gcc reports an error when the code is compiled with -Wmissing-format-attribute.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

qtest.c

@@ -156,7 +156,8 @@ static void qtest_send_prefix(CharDriverState *chr)
             tv.tv_sec, tv.tv_usec);
 }
 
-static void qtest_send(CharDriverState *chr, const char *fmt, ...)
+static void GCC_FMT_ATTR(2, 3) qtest_send(CharDriverState *chr,
+                                          const char *fmt, ...)
 {
     va_list ap;
     char buffer[1024];