Do not remove system bridge virbr0

lib/vagrant-lxc/action/gc_private_network_bridges.rb

@@ -35,7 +35,7 @@ module Vagrant
 
             if ! driver.bridge_is_in_use?(bridge)
               env[:ui].info I18n.t("vagrant_lxc.messages.remove_bridge", name: bridge)
-              if bridge != "lxcbr0"
+              unless ['lxcbr0', 'virbr0'].include? bridge
                 driver.remove_bridge(bridge)
               end
             end

lib/vagrant-lxc/driver.rb

@@ -172,7 +172,7 @@ module Vagrant
       end
 
       def remove_bridge(bridge_name)
-        if bridge_name == "lxcbr0"
+        if ['lxcbr0', 'virbr0'].include? bridge_name
            @logger.info "Skipping removal system bridge #{bridge_name}"
            return
         end