qemu: Enable configuration of HPT resizing for pSeries guests

Most of the time it's okay to leave this up to negotiation between
the guest and the host, but in some situations it can be useful to
manually decide the behavior, especially to enforce its availability.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1308743

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>

docs/schemas/domaincommon.rng

@@ -4736,6 +4736,9 @@
           <optional>
             <ref name="ioapic"/>
           </optional>
+          <optional>
+            <ref name="hpt"/>
+          </optional>
         </interleave>
       </element>
     </optional>
@@ -4924,6 +4927,18 @@
     </element>
   </define>
 
+  <define name="hpt">
+    <element name="hpt">
+      <attribute name="resizing">
+        <choice>
+          <value>enabled</value>
+          <value>disabled</value>
+          <value>required</value>
+        </choice>
+      </attribute>
+    </element>
+  </define>
+
   <define name="address">
     <element name="address">
       <choice>