Use staged APEXes for compilation

Add a new VM config file to be used when we want staged APEXes.

Add an option to compos_key_cmd to use staged APEXes.

Modify compos_client to allow the VM config to be overridden, and use
it to select staged APEXes in start_pending_instance.

Bug: 205296305
Test: stage an APEX, run composd_cmd staged-apex-compile
Change-Id: I379416b9798856d41492b32f6c0042262123a43b
diff --git a/compos/verify_key/verify_key.rs b/compos/verify_key/verify_key.rs
index 945acb4..e0ed5e5 100644
--- a/compos/verify_key/verify_key.rs
+++ b/compos/verify_key/verify_key.rs
@@ -105,8 +105,11 @@
     let instance_image = File::open(instance_image).context("Failed to open instance image")?;
 
     let virtualization_service = VmInstance::connect_to_virtualization_service()?;
-    let vm_instance =
-        VmInstance::start(&*virtualization_service, instance_image, &VmParameters { debug_mode })?;
+    let vm_instance = VmInstance::start(
+        &*virtualization_service,
+        instance_image,
+        &VmParameters { debug_mode, ..Default::default() },
+    )?;
     let service = vm_instance.get_service()?;
 
     let result = service.verifySigningKey(&blob, &public_key).context("Verifying signing key")?;