Avoid setting cpu element on unsupported architectures (#1633)

The CPU element to manage the mode, model, features (including nested),
is only available on some architectures. To allow this plugin to
generate XML valid for other architectures such as RISC-V, the CPU
element needs to be optional and only enabled when the architecture
specified supports it.

Include checks in the validation section to help prevent the setting of
an unsupported architecture with any of the CPU features that require
the CPU element to be available.

Fixes: #1538

spec/unit/action/start_domain_spec.rb

@@ -161,6 +161,71 @@ describe VagrantPlugins::ProviderLibvirt::Action::StartDomain do
       end
     end
 
+    context 'cpu' do
+      let(:test_file) { 'existing.xml' }
+      let(:updated_domain_xml) {
+        new_xml = domain_xml.dup
+        new_xml.gsub!(
+          /<cpu .*\/>/,
+          <<-EOF
+          <cpu check='partial' mode='custom'>
+            <model fallback='allow'>Haswell</model>
+            <feature name='vmx' policy='optional'/>
+            <feature name='svm' policy='optional'/>
+          </cpu>
+          EOF
+        )
+        new_xml
+      }
+      let(:vagrantfile_providerconfig) {
+        <<-EOF
+        libvirt.cpu_mode = 'custom'
+        libvirt.cpu_model = 'Haswell'
+        libvirt.nested = true
+        EOF
+      }
+
+      it 'should set cpu related settings when changed' do
+        expect(ui).to_not receive(:warn)
+        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(libvirt_domain).to receive(:autostart=)
+        expect(domain).to receive(:start)
+
+        expect(subject.call(env)).to be_nil
+      end
+
+      let(:domain_xml_no_cpu) {
+        new_xml = domain_xml.dup
+        new_xml.gsub!(/<cpu .*\/>/, '')
+        new_xml
+      }
+      let(:updated_domain_xml_new_cpu) {
+        new_xml = domain_xml.dup
+        new_xml.gsub!(
+          /<cpu .*\/>/,
+          <<-EOF
+          <cpu mode='custom'>
+            <model fallback='allow'>Haswell</model>
+            <feature name='vmx' policy='optional'/>
+            <feature name='svm' policy='optional'/>
+          </cpu>
+          EOF
+        )
+        new_xml
+      }
+
+      it 'should add cpu settings if not already present' do
+        expect(ui).to_not receive(:warn)
+        expect(connection).to receive(:define_domain).and_return(libvirt_domain)
+        expect(libvirt_domain).to receive(:xml_desc).and_return(domain_xml_no_cpu, updated_domain_xml_new_cpu)
+        expect(libvirt_domain).to receive(:autostart=)
+        expect(domain).to receive(:start)
+
+        expect(subject.call(env)).to be_nil
+      end
+    end
+
     context 'graphics' do
       context 'autoport not disabled' do
         let(:test_file) { 'existing.xml' }