[NET] front: Add SG support

[NET] front: Add SG support

This patch adds scatter-and-gather support to the frontend.  It also
advertises this fact through xenbus so that the backend can detect
this and send through SG requests only if it is supported.

SG support is required to support skb's larger than one page.  This
in turn is needed for either jumbo MTU or TSO.  One of these is
required to bring local networking performance up to a level that
is acceptable.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>