conf: add <ioapic driver> to <features>

Add a new <ioapic> element with a driver attribute.

Possible values are qemu and kvm. With 'qemu', the I/O
APIC can be put in the userspace even for KVM domains.

https://bugzilla.redhat.com/show_bug.cgi?id=1427005

docs/schemas/domaincommon.rng

@@ -4569,6 +4569,9 @@
               </optional>
             </element>
           </optional>
+          <optional>
+            <ref name="ioapic"/>
+          </optional>
         </interleave>
       </element>
     </optional>
@@ -4747,6 +4750,18 @@
     </element>
   </define>
 
+  <define name="ioapic">
+    <element name="ioapic">
+      <attribute name="driver">
+        <choice>
+          <value>qemu</value>
+          <value>kvm</value>
+        </choice>
+      </attribute>
+      <empty/>
+    </element>
+  </define>
+
   <define name="address">
     <element name="address">
       <choice>