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/main.rs b/vm/src/main.rs
index 0af9791..14db27c 100644
--- a/vm/src/main.rs
+++ b/vm/src/main.rs
@@ -108,6 +108,7 @@
     vendor: Option<PathBuf>,
 
     /// SysFS nodes of devices to assign to VM
+    #[cfg(device_assignment)]
     #[arg(long)]
     devices: Vec<PathBuf>,
 }
@@ -132,6 +133,16 @@
     fn vendor(&self) -> Option<PathBuf> {
         None
     }
+
+    #[cfg(device_assignment)]
+    fn devices(&self) -> &Vec<PathBuf> {
+        &self.devices
+    }
+
+    #[cfg(not(device_assignment))]
+    fn devices(&self) -> Vec<PathBuf> {
+        Vec::new()
+    }
 }
 
 #[derive(Args)]