XenVif #46

Revert commit 3714613096b1bb7b7a76e9d360106ea19208f243
Revert commit 80ace57b353764c9165673e021c2a6cf7616a3bb
Re-fix issues uncovered by previous reversions

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #74 from benchalmers/xenvif46

XenVif #46