tests/qtest: add test for querying balloon guest stats

This test would have identified the crash caused by the addition of new
balloon stats fields.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20241129135507.699030-4-berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

tests/qtest/virtio-balloon-test.c

@@ -8,6 +8,7 @@
 
 #include "qemu/osdep.h"
 #include "libqtest.h"
+#include "standard-headers/linux/virtio_balloon.h"
 
 /*
  * https://gitlab.com/qemu-project/qemu/-/issues/2576
@@ -26,11 +27,30 @@ static void oss_fuzz_71649(void)
     qtest_quit(s);
 }
 
+static void query_stats(void)
+{
+    QTestState *s = qtest_init("-device virtio-balloon,id=balloon"
+                               " -nodefaults");
+    QDict *ret = qtest_qmp_assert_success_ref(
+        s,
+        "{ 'execute': 'qom-get', 'arguments': "     \
+        "{ 'path': '/machine/peripheral/balloon', " \
+        "  'property': 'guest-stats' } }");
+    QDict *stats = qdict_get_qdict(ret, "stats");
+
+    /* We expect 1 entry in the dict for each known kernel stat */
+    assert(qdict_size(stats) == VIRTIO_BALLOON_S_NR);
+
+    qobject_unref(ret);
+    qtest_quit(s);
+}
+
 int main(int argc, char **argv)
 {
     g_test_init(&argc, &argv, NULL);
 
     qtest_add_func("virtio-balloon/oss_fuzz_71649", oss_fuzz_71649);
+    qtest_add_func("virtio-balloon/query-stats", query_stats);
 
     return g_test_run();
 }