core: clean up machine directory when id = nil

lib/vagrant/machine.rb

@@ -207,6 +207,12 @@ module Vagrant
       else
         # Delete the file, since the machine is now destroyed
         id_file.delete if id_file.file?
+
+        # Delete the entire data directory contents since all state
+        # associated with the VM is now gone.
+        @data_dir.children.each do |child|
+          child.rmtree
+        end
       end
 
       # Store the ID locally