Add support for spinlocks hyperv feature

spec/unit/templates/domain_all_settings.xml

@@ -34,6 +34,7 @@
     <pae/>
     <hyperv>
       <BBB state='on' />
+      <spinlocks state='on' retries='4096' />
     </hyperv>
   </features>
   <clock offset='variable'>

spec/unit/templates/domain_spec.rb

@@ -43,6 +43,7 @@ describe 'templates/domain' do
       domain.clock_offset = 'variable'
       domain.clock_timer(name: 't1')
       domain.clock_timer(name: 't2', track: 'b', tickpolicy: 'c', frequency: 'd', mode: 'e',  present: 'yes')
+      domain.hyperv_feature(name: 'spinlocks', state: 'on', retries: '4096')
       domain.cputopology(sockets: '1', cores: '3', threads: '2')
       domain.machine_type = 'pc-compatible'
       domain.machine_arch = 'x86_64'