XenVif #52

Kick data/updated after writing attr/ethX keys

For some reason best known to itself XAPI relies on us writing to
data/updated after modifying xenstore keys under attr/ethX to notice
that things have changed. Go figure.

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #85 from benchalmers/xv52

XenVif #52

Approved-By: Ben.Chalmers@citrix.com