doc: add virtio input element in config doc
add virtio input element in the description of elements for
launch XMLs in config doc.
@@ -369,6 +369,10 @@ current scenario has:
Input format:
``[@]stdio|tty|pty|sock:portname[=portpath][,[@]stdio|tty|pty:portname[=portpath]]``.
+``input`` (a child node of ``virtio_devices``):
+ The virtio input device setting.
+ Input format: ``/dev/input/eventX[,serial]``.
+
``cpu_affinity``:
List of pCPU that this VM's vCPUs are pinned to.
