[CA-134598] Store the (partial) tag along with the XENVBD_REQUEST.
Migrate and a bus/target reset will re-queue any existing requests, but not
reset the request tagging correctly, and will eventually run out of free tags.

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Merge pull request #20 from OwenSmith/master

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