Make SSH for forwarded ports write to /dev/null

If ssh inherits stdout from vagrant, then when you pipe vagrants output
to another program ssh will also use that pipe. The receiving program
will hang waiting on ssh to exit. To make reasoning about things easier,
I have ruby start ssh directly instead of via a shell. Then I have
spawn set stdin/stdout to /dev/null.

lib/vagrant-libvirt/action/forward_ports.rb

@@ -82,11 +82,12 @@ module VagrantPlugins
             -L #{host_ip}:#{host_port}:#{guest_ip}:#{guest_port}
             -N
           ).join(' ')
-          ssh_cmd = "ssh $(vagrant ssh-config #{machine}"\
-              "| awk '{print \" -o \"$1\"=\"$2}') #{params} 2>/dev/null"
+          # TODO get options without shelling out
+          options = `vagrant ssh-config #{machine} | awk '{printf " -o "$1"="$2}'`
+          ssh_cmd = "ssh #{options} #{params}"
 
           @logger.debug "Forwarding port with `#{ssh_cmd}`"
-          spawn ssh_cmd
+          spawn(ssh_cmd,  [:out, :err] => '/dev/null')
         end
 
         def store_ssh_pid(host_port, ssh_pid)
@@ -124,7 +125,7 @@ module VagrantPlugins
             ssh_pids.each do |pid|
               next unless ssh_pid?(pid)
               @logger.debug "Killing pid #{pid}"
-              system "pkill -TERM -P #{pid}"
+              system "kill #{pid}"
             end
 
             @logger.info 'Removing ssh pid files'