core: make sure any Subprocess exits no matter what

lib/vagrant/util/subprocess.rb

@@ -203,6 +203,11 @@ module Vagrant
 
         # Return an exit status container
         return Result.new(process.exit_code, io_data[:stdout], io_data[:stderr])
+      ensure
+        if process && !process.exited?
+          # Make sure no matter what happens, the process exits
+          process.stop(2)
+        end
       end
 
       protected