tests/qtest/migration-test: Fix leaks in calc_dirtyrate_ready()

In calc_dirtyrate_ready() we g_strdup() a string but then never free it:

Direct leak of 19 byte(s) in 2 object(s) allocated from:
    #0 0x55ead613413e in malloc (/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/asan/tests/qtest/migration-test+0x22f13e) (BuildId: e7cd5c37b2987a1af682b43ee5240b98bb316737)
    #1 0x7f7a13d39738 in g_malloc debian/build/deb/../../../glib/gmem.c:128:13
    #2 0x7f7a13d4e583 in g_strdup debian/build/deb/../../../glib/gstrfuncs.c:361:17
    #3 0x55ead6266f48 in calc_dirtyrate_ready tests/qtest/migration-test.c:3409:14
    #4 0x55ead62669fe in wait_for_calc_dirtyrate_complete tests/qtest/migration-test.c:3422:13
    #5 0x55ead6253df7 in test_vcpu_dirty_limit tests/qtest/migration-test.c:3562:9
    #6 0x55ead626a407 in migration_test_wrapper tests/qtest/migration-helpers.c:456:5

We also fail to unref the QMP rsp_return, so we leak that also.

Rather than duplicating the string, use the in-place value from
the qdict, and then unref the qdict.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Fabiano Rosas <[email protected]>
Signed-off-by: Fabiano Rosas <[email protected]>

tests/qtest/migration-test.c

@@ -3318,15 +3318,18 @@ static QDict *query_vcpu_dirty_limit(QTestState *who)
 static bool calc_dirtyrate_ready(QTestState *who)
 {
     QDict *rsp_return;
-    gchar *status;
+    const char *status;
+    bool ready;
 
     rsp_return = query_dirty_rate(who);
     g_assert(rsp_return);
 
-    status = g_strdup(qdict_get_str(rsp_return, "status"));
+    status = qdict_get_str(rsp_return, "status");
     g_assert(status);
+    ready = g_strcmp0(status, "measuring");
+    qobject_unref(rsp_return);
 
-    return g_strcmp0(status, "measuring");
+    return ready;
 }
 
 static void wait_for_calc_dirtyrate_complete(QTestState *who,