guest: convert_to_vnc: optionally add qemu-vdagent

Disabled by default, but maybe we turn it on in the future
when qemu-vdagent supports migration.

Signed-off-by: Cole Robinson <crobinso@redhat.com>

tests/data/xmlparse/convert-to-vnc-spice-manyopts-out.xml

@@ -18,6 +18,13 @@
         <acceleration accel3d="yes"/>
       </model>
     </video>
+    <channel type="qemu-vdagent">
+      <source>
+        <clipboard copypaste="no"/>
+        <mouse mode="client"/>
+      </source>
+      <target type="virtio" name="com.redhat.spice.0"/>
+    </channel>
     <graphics type="egl-headless">
       <gl rendernode="/dev/my/rendernode"/>
     </graphics>