Add support for configuring memballoon-related settings (#1083)

Allow configuration of various memballoon-related settings. It was 
discovered that it may be needed to be able to control the memballoon's 
PCI slot/bus location in order to prevent it from conflicting with 
other explicit PCI location assignments. For example when configuring 
the management network NIC to go to slot 0x05, libvirt would try to put 
the memballoon there as well and resulting in a fatal error.

spec/unit/templates/domain_spec.rb

@@ -129,4 +129,54 @@ describe 'templates/domain' do
       expect(domain.to_xml('domain')).to eq xml_expected
     end
   end
+
+  context 'memballoon' do
+    context 'default' do
+      it 'renders without specifying the xml tag' do
+        domain.finalize!
+
+        expect(domain.to_xml('domain')).to_not match(/memballoon/)
+      end
+    end
+
+    context 'memballon enabled' do
+      before do
+        domain.memballoon_enabled = true
+      end
+
+      it 'renders with memballon element' do
+        domain.finalize!
+
+        expect(domain.to_xml('domain')).to match(/<memballoon model='virtio'>/)
+        expect(domain.to_xml('domain')).to match(/<address type='pci' domain='0x0000' bus='0x00' slot='0x0f' function='0x0'\/>/)
+      end
+
+      context 'all settings specified' do
+        before do
+          domain.memballoon_model = "virtio-non-transitional"
+          domain.memballoon_pci_bus = "0x01"
+          domain.memballoon_pci_slot = "0x05"
+        end
+
+        it 'renders with specified values' do
+          domain.finalize!
+
+          expect(domain.to_xml('domain')).to match(/<memballoon model='virtio-non-transitional'>/)
+          expect(domain.to_xml('domain')).to match(/<address type='pci' domain='0x0000' bus='0x01' slot='0x05' function='0x0'\/>/)
+        end
+      end
+    end
+
+    context 'memballon disabled' do
+      before do
+        domain.memballoon_enabled = false
+      end
+
+      it 'renders the memballoon element with model none' do
+        domain.finalize!
+
+        expect(domain.to_xml('domain')).to match(/<memballoon model='none'\/>/)
+      end
+    end
+  end
 end