[NET] front: Add TSO support

[NET] front: Add TSO support

This patch adds TCP Segmentation Offload (TSO) support to the
frontend.
It also advertises this fact through xenbus so that the frontend can
detect this and send through TSO requests only if it is supported.

This is done using an extra request slot which is indicated by a flag
in the first slot.  In future checksum offload can be done in the same
way.

Even though only TSO is supported for now the code actually supports
GSO so it can be applied to any other protocol.  The only missing bit
is the detection of host support for a specific GSO protocol.  Once
that is added we can advertise all supported protocols to the guest.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Disable for now, as in domU->dom0 direction.

Signed-off-by: Keir Fraser <keir@xensource.com>