conf: parse and format virtio input bus in domain XML

To be used by the family of virtio input devices:

<input type='mouse' bus='virtio'/>
<input type='tablet' bus='virtio'/>
<input type='keyboard' bus='virtio'/>

https://bugzilla.redhat.com/show_bug.cgi?id=1231114

docs/formatdomain.html.in

@@ -4801,6 +4801,9 @@ qemu-kvm -net nic,model=? /dev/null
   <devices>
     <input type='mouse' bus='usb'/>
     <input type='keyboard' bus='usb'/>
+    <input type='mouse' bus='virtio'/>
+    <input type='keyboard' bus='virtio'/>
+    <input type='tablet' bus='virtio'/>
   </devices>
   ...</pre>
 
@@ -4812,7 +4815,8 @@ qemu-kvm -net nic,model=? /dev/null
         The tablet provides absolute cursor movement,
         while the mouse uses relative movement. The optional
         <code>bus</code> attribute can be used to refine the exact device type.
-        It takes values "xen" (paravirtualized), "ps2" and "usb".</dd>
+        It takes values "xen" (paravirtualized), "ps2" and "usb" or
+        (<span class="since">since 1.3.0</span>) "virtio".</dd>
     </dl>
 
     <p>

docs/schemas/domaincommon.rng

@@ -3592,6 +3592,7 @@
             <value>ps2</value>
             <value>usb</value>
             <value>xen</value>
+            <value>virtio</value>
           </choice>
         </attribute>
       </optional>