blktap2: make protocol specific usage of shared sring explicit

blktap2: make protocol specific usage of shared sring explicit

I don't think protocol specific data really belongs in this header
but since it is already there and we seem to be stuck with it let's at
least make the users explicit lest people get caught out by future new
fields moving the pad field around.

This is the Xen portion of this change. The kernel portion will be
sent separately. There is no dependency between the two.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Daniel Stodden <daniel.stodden@citrix.com>
Cc: Dongxiao Xu <dongxiao.xu@intel.com>