Pass a tag to blkif_request_t.id instead of a raw pointer.
Validate blkif_response_t.id is a correctly formatted tag before
retrieving the appropriate PXENVBD_REQUEST pointer.

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Zero ring slot after processing response

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Merge pull request #12 from OwenSmith/tags

Dont pass raw pointers to backend via req.id