Handle different attribute and element ordering (#1592)

Normalise the XML to ensure the attributes for both documents have the
same ordering to prevent excessive noise when differences are detected.
Additionally sort various elements based on attributes that make
ordering irrelevant to allow for simpler comparison using xmlsimple.

Closes: #1583

spec/unit/action/start_domain_spec.rb

@@ -74,6 +74,26 @@ describe VagrantPlugins::ProviderLibvirt::Action::StartDomain do
       end
     end
 
+    context 'when xml elements and attributes reordered' do
+      let(:test_file) { 'existing.xml' }
+      let(:updated_test_file) { 'existing_reordered.xml' }
+      let(:vagrantfile_providerconfig) do
+        <<-EOF
+        libvirt.cpu_mode = "host-passthrough"
+        EOF
+      end
+
+      it 'should correctly detect the domain was updated' do
+        expect(ui).to_not receive(:warn)
+        expect(libvirt_domain).to receive(:autostart=)
+        expect(connection).to receive(:define_domain).and_return(libvirt_domain)
+        expect(libvirt_domain).to receive(:xml_desc).and_return(domain_xml, updated_domain_xml)
+        expect(domain).to receive(:start)
+
+        expect(subject.call(env)).to be_nil
+      end
+    end
+
     context 'when xml not applied' do
       let(:test_file) { 'default_with_different_formatting.xml' }
       let(:updated_domain_xml) {

spec/unit/action/start_domain_spec/existing_reordered.xml

@@ -0,0 +1,62 @@
+<domain type='qemu'>
+  <name>vagrant-libvirt_default</name>
+  <uuid>881a931b-0110-4d10-81aa-47a1a19f5726</uuid>
+  <description>Source: /home/test/vagrant-libvirt/Vagrantfile</description>
+  <memory unit='KiB'>2097152</memory>
+  <currentMemory unit='KiB'>2097152</currentMemory>
+  <vcpu placement='static'>2</vcpu>
+  <os>
+    <type arch='x86_64' machine='pc-i440fx-6.0'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <features>
+    <acpi/>
+    <apic/>
+    <pae/>
+  </features>
+  <cpu check='partial' mode='host-passthrough'/>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <disk device='disk' type='file'>
+      <driver name='qemu' type='qcow2'/>
+      <source file='/var/lib/libvirt/images/vagrant-libvirt_default.img'/>
+      <target bus='virtio' dev='vda'/>
+      <address bus='0x00' domain='0x0000' function='0x0' slot='0x03' type='pci'/>
+    </disk>
+    <controller index='0' model='pci-root' type='pci'/>
+    <controller index='0' model='piix3-uhci' type='usb'>
+      <address bus='0x00' domain='0x0000' function='0x2' slot='0x01' type='pci'/>
+    </controller>
+    <interface type='network'>
+      <mac address='52:54:00:7d:14:0e'/>
+      <source network='vagrant-libvirt'/>
+      <model type='virtio'/>
+      <address bus='0x00' domain='0x0000' function='0x0' slot='0x05' type='pci'/>
+    </interface>
+    <serial type='pty'>
+      <target port='0' type='isa-serial'>
+        <model name='isa-serial'/>
+      </target>
+    </serial>
+    <console type='pty'>
+      <target port='0' type='serial'/>
+    </console>
+    <input bus='ps2' type='mouse'/>
+    <input bus='ps2' type='keyboard'/>
+    <graphics autoport='yes' keymap='en-us' listen='127.0.0.1' port='-1' type='vnc'>
+      <listen address='127.0.0.1' type='address'/>
+    </graphics>
+    <audio id='1' type='none'/>
+    <video>
+      <model heads='1' primary='yes' type='cirrus' vram='16384'/>
+      <address bus='0x00' domain='0x0000' function='0x0' slot='0x02' type='pci'/>
+    </video>
+    <memballoon model='virtio'>
+      <address bus='0x00' domain='0x0000' function='0x0' slot='0x04' type='pci'/>
+    </memballoon>
+  </devices>
+</domain>