qtest: fix a memory leak

Spotted by ASAN.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

qtest.c

@@ -240,6 +240,7 @@ static void GCC_FMT_ATTR(2, 3) qtest_sendf(CharBackend *chr,
     va_start(ap, fmt);
     buffer = g_strdup_vprintf(fmt, ap);
     qtest_send(chr, buffer);
+    g_free(buffer);
     va_end(ap);
 }