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/vm/src/run.rs b/vm/src/run.rs
index 1ba9dec..84c3af6 100644
--- a/vm/src/run.rs
+++ b/vm/src/run.rs
@@ -130,7 +130,7 @@
         vendorImage: vendor,
         devices: config
             .microdroid
-            .devices
+            .devices()
             .iter()
             .map(|x| {
                 x.to_str().map(String::from).ok_or(anyhow!("Failed to convert {x:?} to String"))