Flag guard vfio_handler

Guard the following on the value of the
RELEASE_AVF_ENABLE_DEVICE_ASSIGNMENT flag:

* vfio_handler binary & vfio_handler.rc in com.android.virt APEX;
* --devices flag in the vm shell binary
* Usage of the CustomConfig.devices field in the
  VirtualMachineAppConfig;

Test: atest MicrodroidTests
Change-Id: I40611b0cd93d17aacd68acf74f442e479a80d488
diff --git a/apex/vfio_handler.rc b/apex/vfio_handler.rc
new file mode 100644
index 0000000..419acef
--- /dev/null
+++ b/apex/vfio_handler.rc
@@ -0,0 +1,20 @@
+# Copyright (C) 2023 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+service vfio_handler /apex/com.android.virt/bin/vfio_handler
+    user root
+    group system
+    interface aidl android.system.virtualizationservice_internal.IVfioHandler
+    disabled
+    oneshot