xen/scsifront: don't request a slot on the ring until request is ready

xen/scsifront: don't request a slot on the ring until request is ready

Instead of requesting a new slot on the ring to the backend early, do
so only after all has been setup for the request to be sent. This
makes error handling easier as we don't need to undo the request id
allocation and ring slot allocation.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
OraBug: 25497392

(cherry picked from commit 3da96be58f2c8aaa86cfe78b16f837e610dfcfe2)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>