Restore handling of disk_device domain setting (#1365)

Re-enable handling of the disk_device domain volume setting to ensure it
can be overridden from the default of vda to a value chosen.

Provide a disk resolver to resolve devices after the box has been downloaded
so that initial devices can be correctly allocated and avoid conflicts with
additional disks added that would otherwise get assigned the same device.

Removes hack for destroy domain when more than one disk, as now devices
in the config are only present if provided by the configuration.

Fixes: #1353

spec/unit/action/create_domain_spec.rb

@@ -33,7 +33,6 @@ describe VagrantPlugins::ProviderLibvirt::Action::CreateDomain do
       allow(connection).to receive(:volumes).and_return(volumes)
       allow(volumes).to receive(:all).and_return([domain_volume])
       allow(domain_volume).to receive(:pool_name).and_return('default')
-      allow(domain_volume).to receive(:[]).with('name').and_return('vagrant-test_default.img')
       allow(domain_volume).to receive(:path).and_return('/var/lib/libvirt/images/vagrant-test_default.img')
       allow(machine).to receive_message_chain("box.name") { 'vagrant-libvirt/test' }
 
@@ -117,6 +116,48 @@ describe VagrantPlugins::ProviderLibvirt::Action::CreateDomain do
             end
           end
         end
+
+        context 'with custom disk device setting' do
+          let(:domain_xml_file) { 'custom_disk_settings.xml' }
+          let(:vagrantfile_providerconfig) {
+            <<-EOF
+              libvirt.disk_device = 'sda'
+            EOF
+          }
+
+          it 'should set the domain device' do
+            expect(ui).to receive(:info).with(/ -- Image\(sda\):.*/)
+            expect(servers).to receive(:create).with(xml: domain_xml).and_return(machine)
+
+            expect(subject.call(env)).to be_nil
+          end
+        end
+
+        context 'with two domain disks' do
+          let(:domain_xml_file) { 'two_disk_settings.xml' }
+          let(:domain_volume_2) { double('domain_volume 2') }
+
+          before do
+            expect(volumes).to receive(:all).and_return([domain_volume])
+            expect(volumes).to receive(:all).and_return([domain_volume_2])
+            expect(domain_volume_2).to receive(:pool_name).and_return('default')
+            expect(domain_volume_2).to receive(:path).and_return('/var/lib/libvirt/images/vagrant-test_default_1.img')
+
+            env[:box_volumes].push({
+              :path=>"/test/box_1.img",
+              :name=>"test_vagrant_box_image_1.1.1_1.img",
+              :virtual_size=> ByteNumber.new(5),
+            })
+          end
+
+          it 'should correctly assign device entries' do
+            expect(ui).to receive(:info).with(/ -- Image\(vda\):.*/)
+            expect(ui).to receive(:info).with(/ -- Image\(vdb\):.*/)
+            expect(servers).to receive(:create).with(xml: domain_xml).and_return(machine)
+
+            expect(subject.call(env)).to be_nil
+          end
+        end
       end
 
       context 'no default pool' do