tests/qtest/migration: Print migration incoming errors

We're currently just asserting when incoming migration fails. Let's
print the error message from QMP as well.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20240104142144.9680-6-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

tests/qtest/migration-helpers.c

@@ -111,6 +111,12 @@ void migrate_incoming_qmp(QTestState *to, const char *uri, const char *fmt, ...)
 
     rsp = qtest_qmp(to, "{ 'execute': 'migrate-incoming', 'arguments': %p}",
                     args);
+
+    if (!qdict_haskey(rsp, "return")) {
+        g_autoptr(GString) s = qobject_to_json_pretty(QOBJECT(rsp), true);
+        g_test_message("%s", s->str);
+    }
+
     g_assert(qdict_haskey(rsp, "return"));
     qobject_unref(rsp);