block: Introduce .bdrv_co_preadv/pwritev BlockDriver function

block: Introduce .bdrv_co_preadv/pwritev BlockDriver function

Many parts of the block layer are already byte granularity. The block
driver interface, however, was still missing an interface that allows
making use of this. This patch introduces a new BlockDriver interface,
which is based on coroutines, vectored, has flags and uses a byte
granularity. This is now the preferred interface for new drivers.

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