add-cow file format core code.

add-cow file format core code. It use block-cache.c as cache code.
It lacks of snapshot_blkdev support.

v17-v18:
1) use error_report, not fprintf.
2) remove version field from header.
3) header_size is MAX(cluster_size, 4096).
4) introduce s->cluster_sectors.
5) use BLKDBG_L2_LOAD/UPDATE.

v16->v17:
1) Use stringify.

v15->v16:
1) Judge if opts is null in add_cow_create function.

Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>

Conflicts:
	block/block-cache.c
without flush