halt: check that machine is running before attempting to clear tmpfiles

Otherwise, running `vagrant halt` on a stopped machine would make us
crash. Particularly annoying in multi-machines environments.

ref #406

lib/vagrant-lxc/action.rb

@@ -125,7 +125,9 @@ module Vagrant
             end
 
             b2.use ClearForwardedPorts
-            b2.use RemoveTemporaryFiles
+            if env[:machine].state.id == :running
+              b2.use RemoveTemporaryFiles
+            end
             b2.use GcPrivateNetworkBridges
             b2.use Builtin::Call, Builtin::GracefulHalt, :stopped, :running do |env2, b3|
               if !env2[:result]