Allow bridges referenced by libvirt networks (#1554)

Permit reference to bridge devices referenced by existing libvirt
networks.

Fixes: #1553

docs/configuration.markdown

@@ -512,6 +512,14 @@ virtual network.
   are listed [here](http://www.libvirt.org/formatdomain.html#elementsNICSDirect).
   Default is 'false'.
 
+Additionally for public networks, to facilitate validating if the device provided
+can be used, vagrant-libvirt will check both the host interfaces visible to libvirt
+and the existing networks for any existing bridge names. While some name patterns are
+automatically excluded as presumed incorrect, if this pattern list is incorrect
+it may be overridden by setting the option:
+* `host_device_exclude_prefixes` - ignore any device starting with any of these
+  string patterns as a valid bridge device for a public network definition.
+
 ### Management Network
 
 vagrant-libvirt uses a private network to perform some management operations on

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)

spec/unit/config_spec.rb

@@ -640,12 +640,20 @@ describe VagrantPlugins::ProviderLibvirt::Config do
         instance_double(Libvirt::Interface),
         instance_double(Libvirt::Interface),
       ] }
+      let(:libvirt_networks) { [
+        instance_double(Libvirt::Network),
+        instance_double(Libvirt::Network),
+      ] }
+      let(:driver) { instance_double(::VagrantPlugins::ProviderLibvirt::Driver) }
       before do
         machine.config.vm.network :public_network, dev: 'eth0', ip: "192.168.2.157"
-        expect(machine).to receive_message_chain('provider.driver.connection.client').and_return(libvirt_client)
-        expect(libvirt_client).to receive(:list_all_interfaces).and_return(host_devices)
+        allow(machine.provider).to receive(:driver).and_return(driver)
+        expect(driver).to receive(:list_host_devices).and_return(host_devices)
+        expect(driver).to receive(:list_networks).and_return(libvirt_networks)
         expect(host_devices[0]).to receive(:name).and_return('eth0')
         expect(host_devices[1]).to receive(:name).and_return('virbr0')
+        expect(libvirt_networks[0]).to receive(:bridge_name).and_return('')
+        expect(libvirt_networks[1]).to receive(:bridge_name).and_return('virbr0')
       end
 
       it 'should validate use of existing device' do