blkif: add indirect descriptors interface to public headers

blkif: add indirect descriptors interface to public headers

Indirect descriptors introduce a new block operation
(BLKIF_OP_INDIRECT) that passes grant references instead of segments
in the request. This grant references are filled with arrays of
blkif_request_segment_aligned, this way we can send more segments in a
request.

This interface is already implemented in Linux >= 3.11.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>