qemu: add support for Hyper-V Enlightened VMCS

QEMU 3.1 supports Hyper-V Enlightened VMCS feature which significantly
speeds up nested Hyper-V on KVM environments.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

tests/qemuxml2xmloutdata/hyperv-off.xml

@@ -24,6 +24,7 @@
       <reenlightenment state='off'/>
       <tlbflush state='off'/>
       <ipi state='off'/>
+      <evmcs state='off'/>
     </hyperv>
   </features>
   <clock offset='utc'/>

tests/qemuxml2xmloutdata/hyperv.xml

@@ -24,6 +24,7 @@
       <reenlightenment state='on'/>
       <tlbflush state='on'/>
       <ipi state='on'/>
+      <evmcs state='on'/>
     </hyperv>
   </features>
   <clock offset='utc'/>