blkverify: Implement bdrv_co_preadv/pwritev/flush

This enables byte granularity requests for blkverify, and at the same
time gets us rid of another user of the BDS-level AIO emulation.

The reference output of a test case must be changed because the
verification failure message reports byte offsets instead of sectors
now.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
