tests/qtest: add assert to catch bad features

No device driver (which is what the qvirtio_ access functions
represent) should be setting UNUSED(30) in the feature space. Although
existing libqos users mask it out lets ensure nothing sneaks through.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220802095010.3330793-20-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tests/qtest/libqos/virtio.c

@@ -101,6 +101,8 @@ uint64_t qvirtio_get_features(QVirtioDevice *d)
 
 void qvirtio_set_features(QVirtioDevice *d, uint64_t features)
 {
+    g_assert(!(features & QVIRTIO_F_BAD_FEATURE));
+
     d->features = features;
     d->bus->set_features(d, features);