After allocation the item is being placed on the list right away.
Consequently it needs to be taken off the list before freeing in the
case xenbus_dev_request_and_reply() failed, as in that case the
callback (xenbus_dev_queue_reply()) is not being called (and if it
was called, it should do both).

Fixes: 5584ea25Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>