xen/netfront: Request no checksum offload

Checksum offload is not plumbed through to the networking stack, so
does not work.  Turn it off by writing the relevant key in xenstore.

It would be better, but more work, to plumb it through.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Merge pull request #7 from ijackson-citrix/netfront-csum-offload

xen/netfront: Request no checksum offload