Update XENVIF

Changes:

Commit ef6df6cfda9a58a1bbe66eb638ef710e749bcd30 by owen.smith
Ensure Gratuitous ARPs and Neighbour Advertisements are sent 3 times.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge pull request #2 from pauldu/master

Update XENVIF