public/io/netif: specify MTU override node

There is currently no documentation to state what MTU a frontend should
adertise to its network stack. It has however long been assumed that the
default value of 1500 is correct.

This patch specifies a mechanism to allow the tools to set the MTU via a
xenstore node in the frontend area and states that the absence of that node
means the frontend should assume an MTU of 1500 octets.

NOTE: The Windows PV frontend has used an MTU sampled from the xenstore
      node specified in this patch.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wl@xen.org>

docs/misc/xenstore-paths.pandoc

@@ -298,6 +298,9 @@ A virtual keyboard device frontend. Described by
 A virtual network device frontend. Described by
 [xen/include/public/io/netif.h][NETIF]
 
+NOTE: ~/device/vif/$DEVID/mtu can be used to inform the frontend of an
+      increased MTU. (The default MTU is 1500 octets).
+
 #### ~/device/vscsi/$DEVID/* []
 
 A virtual scsi device frontend. Described by

xen/include/public/io/netif.h

@@ -204,6 +204,18 @@
  * present).
  */
 
+/*
+ * MTU
+ * ===
+ *
+ * The toolstack may set a value of MTU for the frontend by setting the
+ * /local/domain/<domid>/device/vif/<vif>/mtu node with the MTU value in
+ * octets. If this node is absent the frontend should assume an MTU value
+ * of 1500 octets. A frontend is also at liberty to ignore this value so
+ * it is only suitable for informing the frontend that a packet payload
+ * >1500 octets is permitted.
+ */
+
 /*
  * Hash types
  * ==========