Include -n option for forwarded port ssh processes

In configurations where ControlMaster is used with ControlPersist
set, not including this option results in "stolen" input.

lib/vagrant-libvirt/action/forward_ports.rb

@@ -120,7 +120,7 @@ module VagrantPlugins
             end
           end
 
-          ssh_cmd << "ssh #{options} #{params}"
+          ssh_cmd << "ssh -n #{options} #{params}"
 
           @logger.debug "Forwarding port with `#{ssh_cmd}`"
           log_file = ssh_forward_log_file(host_ip, host_port,