qemu: support kvm-pv-ipi off

QEMU version 3.1 introduced PV_SEND_IPI CPUID feature bit under
commit 7f710c32bb8 (target-i386: adds PV_SEND_IPI CPUID feature bit).

This patch adds a new KVM feature 'pv-ipi' to disable this feature
(enabled by default). Newer CPU platform (Ex, AMD Zen2) supports
hardware accelation for IPI in guest, to use this feature to get
better performance in some scenarios. Detailed about the discussion:
  https://lkml.org/lkml/2021/10/20/423

To disable kvm-pv-ipi and have libvirt add "-cpu host,kvm-pv-ipi=off"
to the QEMU command line, the following XML code needs to be added to the
guest's domain description:

  <features>
    <kvm>
      <pv-ipi state='off'/>
    </kvm>
  </features>

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

docs/schemas/domaincommon.rng

@@ -7177,6 +7177,11 @@
             <ref name="featurestate"/>
           </element>
         </optional>
+        <optional>
+          <element name="pv-ipi">
+            <ref name="featurestate"/>
+          </element>
+        </optional>
       </interleave>
     </element>
   </define>