tests: Add tests for kvm-poll-control feature

Update the KVM feature tests for QEMU's kvm-poll-control performance
hint.

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/qemuxml2xmloutdata/kvm-features.xml

@@ -13,6 +13,7 @@
     <kvm>
       <hidden state='on'/>
       <hint-dedicated state='on'/>
+      <poll-control state='on'/>
     </kvm>
   </features>
   <cpu mode='host-passthrough' check='none' migratable='off'/>