tests/qtest: migration-test: Fix [-Werror=format-overflow=] build warning

When tmpfs is NULL, a build warning is seen with GCC 9.3.0.
It's strange that GCC 11.2.0 on Ubuntu 22.04 does not catch this,
neither did the QEMU CI.

While we are here, improve the error message as well.

Reported-by: Shengjiang Wu <shengjiang.wu@windriver.com>
Fixes: e5553c1b8d ("tests/qtest: migration-test: Avoid using hardcoded /tmp")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221017132023.2228641-1-bmeng.cn@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

tests/qtest/migration-test.c

@@ -2481,8 +2481,8 @@ int main(int argc, char **argv)
 
     tmpfs = g_dir_make_tmp("migration-test-XXXXXX", &err);
     if (!tmpfs) {
-        g_test_message("g_dir_make_tmp on path (%s): %s", tmpfs,
-                       err->message);
+        g_test_message("Can't create temporary directory in %s: %s",
+                       g_get_tmp_dir(), err->message);
     }
     g_assert(tmpfs);