varlink: add enum types for class and whom fields in io.systemd.Machine

Convert the class field (Register input, List output) from plain string
to MachineClass enum type, and the whom field (Kill input) to KillWhom
enum type.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

src/test/meson.build

@@ -503,6 +503,10 @@ executables += [
         core_test_template + {
                 'sources' : files('test-varlink-idl-manager.c'),
         },
+        test_template + {
+                'sources' : files('test-varlink-idl-machine.c'),
+                'objects' : ['systemd-machined'],
+        },
         test_template + {
                 'sources' : files('test-watchdog.c'),
                 'type' : 'unsafe',

src/test/test-varlink-idl-machine.c

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "machine.h"
+#include "tests.h"
+#include "test-varlink-idl-util.h"
+#include "varlink-io.systemd.Machine.h"
+
+TEST(machine_enums_idl) {
+        TEST_IDL_ENUM(MachineClass, machine_class, vl_type_MachineClass);
+        TEST_IDL_ENUM(KillWhom, kill_whom, vl_type_KillWhom);
+}
+
+DEFINE_TEST_MAIN(LOG_DEBUG);