XENVBD 18

[CA-134598] Store the (partial) tag along with the XENVBD_REQUEST

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #53 from benchalmers/xvbd18

XENVBD 18

Approved-By: Ben Chalmers <Ben.Chalmers@citrix.com>