Add --prefer-staged to composd_cmd test-compile

This change makes it eaiser to expend the composd_cmd CLI to various
debugging options, starting with using staged APEX.

Bug: 220341414
Test: Stage an APEX. Run test-compile with and without --prefer-staged
      # See VS' debug log with corresponding APEX path
Change-Id: I155681bc74f716e75ababb439e6bf48f43e0ce67
diff --git a/compos/composd/src/instance_manager.rs b/compos/composd/src/instance_manager.rs
index 848fc8c..f1289e8 100644
--- a/compos/composd/src/instance_manager.rs
+++ b/compos/composd/src/instance_manager.rs
@@ -48,9 +48,12 @@
         self.start_instance(CURRENT_INSTANCE_DIR, vm_parameters)
     }
 
-    pub fn start_test_instance(&self) -> Result<Arc<CompOsInstance>> {
+    pub fn start_test_instance(&self, prefer_staged: bool) -> Result<Arc<CompOsInstance>> {
         let mut vm_parameters = new_vm_parameters()?;
         vm_parameters.debug_mode = true;
+        if prefer_staged {
+            vm_parameters.config_path = Some(PREFER_STAGED_VM_CONFIG_PATH.to_owned());
+        }
         self.start_instance(TEST_INSTANCE_DIR, vm_parameters)
     }