[Fix] forward_ports.rb: no implicit conversion of String into Array (#1706)

Fixes a TypeError due to attempting to add a string to an array instead of
appending as a new element when gateway ports are set.

lib/vagrant-libvirt/action/forward_ports.rb

@@ -93,7 +93,7 @@ module VagrantPlugins
             -N
             #{ssh_info[:host]}
           )
-          params += '-g' if gateway_ports
+          params <<= '-g' if gateway_ports
 
           options = (%W(
             User=#{ssh_info[:username]}