Fix forward_ports with ruby 3.0 (#1242)

Fedora 24 has moved to ruby 3.0, which triggers an exception in 
vagrant-libvirt when installing it with vagrant provided by the distro.

This is caused by a chance in ruby 3.0 that can be applied to earlier
versions without any down sides:
https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

lib/vagrant-libvirt/action/forward_ports.rb

@@ -44,7 +44,7 @@ module VagrantPlugins
 
             @env[:ui].info(I18n.t(
                              'vagrant.actions.vm.forward_ports.forwarding_entry',
-                             message_attributes
+                             **message_attributes
             ))
 
             if fp[:protocol] == 'udp'