Allow bridges referenced by libvirt networks (#1554)

Permit reference to bridge devices referenced by existing libvirt
networks.

Fixes: #1553

lib/vagrant-libvirt/config.rb

@@ -1213,7 +1213,11 @@ module VagrantPlugins
 
       def host_devices(machine)
         @host_devices ||= begin
-          machine.provider.driver.connection.client.list_all_interfaces().map { |iface| iface.name }.uniq.select do |dev|
+          (
+            machine.provider.driver.list_host_devices.map { |iface| iface.name } +
+            machine.provider.driver.list_networks.map { |net| net.bridge_name }
+          ).uniq.select do |dev|
+            next if dev.empty?
             dev != "lo" && !@host_device_exclude_prefixes.any? { |exclude| dev.start_with?(exclude) }
           end
         end

lib/vagrant-libvirt/driver.rb

@@ -201,6 +201,14 @@ module VagrantPlugins
         state
       end
 
+      def list_host_devices
+        @connection.client.list_all_interfaces
+      end
+
+      def list_networks
+        @connection.client.list_all_networks
+      end
+
       private
 
       def get_ipaddress_from_system(mac)