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/Android.bp b/Android.bp
index 550a6be..9c17c7f 100644
--- a/Android.bp
+++ b/Android.bp
@@ -22,6 +22,7 @@
     module_type: "rust_defaults",
     config_namespace: "ANDROID",
     bool_variables: [
+        "release_avf_enable_device_assignment",
         "release_avf_enable_dice_changes",
         "release_avf_enable_llpvm_changes",
         "release_avf_enable_multi_tenant_microdroid_vm",
@@ -36,6 +37,9 @@
 avf_flag_aware_rust_defaults {
     name: "avf_build_flags_rust",
     soong_config_variables: {
+        release_avf_enable_device_assignment: {
+            cfgs: ["device_assignment"],
+        },
         release_avf_enable_dice_changes: {
             cfgs: ["dice_changes"],
         },