Validation on `config.vm.boot_mode`

lib/vagrant/config/vm.rb

@@ -103,6 +103,8 @@ module Vagrant
                        :path => options[:hostpath])
           end
         end
+
+        errors.add("vagrant.config.vm.boot_mode_invalid") if ![:vrdp, :gui].include?(boot_mode.to_sym)
       end
     end
   end

templates/locales/en.yml

@@ -101,6 +101,7 @@ en:
         private_key_missing: "`private_key_path` file must exist: %{path}"
       vm:
         shared_folder_hostpath_missing: "Shared folder host path for '%{name}' doesn't exist: %{path}"
+        boot_mode_invalid: "Boot mode must be one of: vrdp or gui"
 
 #-------------------------------------------------------------------------------
 # Translations for commands. e.g. `vagrant x`