block: make bdrv_co_do_write_zeroes stricter in producing aligned requests

block: make bdrv_co_do_write_zeroes stricter in producing aligned requests

Right now, bdrv_co_do_write_zeroes will only try to align the
beginning of the request.  However, it is simpler for many
formats to expect the block layer to separate both the head *and*
the tail.  This makes sure that the format's bdrv_co_write_zeroes
function will be called with aligned sector_num and nb_sectors for
the bulk of the request.

Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Peter Lieven <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>