Add vm_shell script

It's a small script that makes it easier to connect to a Microdroid VM
running in the Android device.

Usage: vm_shell [cid]

If cid is omitted it displays the CIDs of available VMs and lets you
select one.

Bug: N/A
Test: m vm_shell and run it
Change-Id: I762f1161024d151a2f9e75fdb217037f1f9a1e1d
diff --git a/apex/Android.bp b/apex/Android.bp
index 923c378..83985cc 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -69,6 +69,9 @@
     ],
     file_contexts: ":com.android.virt-file_contexts",
     canned_fs_config: "canned_fs_config",
+    host_required: [
+        "vm_shell",
+    ],
 }
 
 apex_key {