system_api: add explicit error code for license issues for Plugin VM

It is beneficial to know whether certain operations failed because of
invalid license or other reasons.

BUG=b:133521686
TEST=Build

Change-Id: I763706e9fefd0a476d7d82808f556911937b7335
Reviewed-on: https://chromium-review.googlesource.com/1660886
Tested-by: Dmitry Torokhov <dtor@chromium.org>
Commit-Ready: Dmitry Torokhov <dtor@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Cr-Mirrored-From: https://chromium.googlesource.com/chromiumos/platform2
Cr-Mirrored-Commit: b1c3be8c490ea9a3a84569282f43238e284b6d71
diff --git a/dbus/vm_plugin_dispatcher/vm_plugin_dispatcher.proto b/dbus/vm_plugin_dispatcher/vm_plugin_dispatcher.proto
index d41a714..506de92 100644
--- a/dbus/vm_plugin_dispatcher/vm_plugin_dispatcher.proto
+++ b/dbus/vm_plugin_dispatcher/vm_plugin_dispatcher.proto
@@ -34,6 +34,8 @@
   VM_SUCCESS = 0;
   // Unspecified error.
   VM_ERR_UNKNOWN = 1;
+  // No valid license.
+  VM_ERR_LIC_NOT_VALID = 2;
 }
 
 // Request to register VM residing at given path.