block: add default implementations for bdrv_co_get_block_status()

block: add default implementations for bdrv_co_get_block_status()

bdrv_co_get_block_status_from_file() and
bdrv_co_get_block_status_from_backing() set *file to bs->file and
bs->backing respectively, so that bdrv_co_get_block_status() can recurse
to them. Future block drivers won't have to duplicate code to implement
this.

Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Manos Pitsidianakis <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>