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

Test: presubmit
Bug: 333730505
Change-Id: I867bb2edeb9f5a285fdee59dae2366e3afeb707d
diff --git a/android/vm/src/main.rs b/android/vm/src/main.rs
index 81ca8fa..110e0ca 100644
--- a/android/vm/src/main.rs
+++ b/android/vm/src/main.rs
@@ -155,10 +155,10 @@
     #[arg(long)]
     devices: Vec<PathBuf>,
 
-    /// Version of GKI to use. If set, use instead of microdroid kernel
-    #[cfg(vendor_modules)]
+    /// Version of OS to use. If not set, defaults to microdroid.
+    /// You can list all available OSes via `vm info` command.
     #[arg(long)]
-    gki: Option<String>,
+    os: Option<String>,
 }
 
 impl MicrodroidConfig {
@@ -172,16 +172,6 @@
         }
     }
 
-    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)] {