Revert "vm tool: rename --gki arg to more generic --os one"

Revert submission 3323102

Reason for revert: DroidMonitor: Potential culprit for http://b/376483022 - verifying through ABTD before revert submission. This is part of the standard investigation process, and does not mean your CL will be reverted.

Reverted changes: /q/submissionid:3323102

Change-Id: Id2923b891929dd695821da09a116a95d16ef8372
diff --git a/android/vm/src/main.rs b/android/vm/src/main.rs
index 110e0ca..81ca8fa 100644
--- a/android/vm/src/main.rs
+++ b/android/vm/src/main.rs
@@ -155,10 +155,10 @@
     #[arg(long)]
     devices: Vec<PathBuf>,
 
-    /// Version of OS to use. If not set, defaults to microdroid.
-    /// You can list all available OSes via `vm info` command.
+    /// Version of GKI to use. If set, use instead of microdroid kernel
+    #[cfg(vendor_modules)]
     #[arg(long)]
-    os: Option<String>,
+    gki: Option<String>,
 }
 
 impl MicrodroidConfig {
@@ -172,6 +172,16 @@
         }
     }
 
+    fn gki(&self) -> Option<&str> {
+        cfg_if::cfg_if! {
+            if #[cfg(vendor_modules)] {
+                self.gki.as_deref()
+            } else {
+                None
+            }
+        }
+    }
+
     fn devices(&self) -> &[PathBuf] {
         cfg_if::cfg_if! {
             if #[cfg(device_assignment)] {