tests: fix test-vmstate leaks

Spotted thanks to ASAN.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

tests/test-vmstate.c

@@ -50,16 +50,20 @@ static QEMUFile *open_test_file(bool write)
 {
     int fd = dup(temp_fd);
     QIOChannel *ioc;
+    QEMUFile *f;
+
     lseek(fd, 0, SEEK_SET);
     if (write) {
         g_assert_cmpint(ftruncate(fd, 0), ==, 0);
     }
     ioc = QIO_CHANNEL(qio_channel_file_new_fd(fd));
     if (write) {
-        return qemu_fopen_channel_output(ioc);
+        f = qemu_fopen_channel_output(ioc);
     } else {
-        return qemu_fopen_channel_input(ioc);
+        f = qemu_fopen_channel_input(ioc);
     }
+    object_unref(OBJECT(ioc));
+    return f;
 }
 
 #define SUCCESS(val) \