block-backend: convert blk_aio_ functions to int64_t bytes paramter

block-backend: convert blk_aio_ functions to int64_t bytes paramter

1. Convert bytes in BlkAioEmAIOCB:
  aio->bytes is only passed to already int64_t interfaces, and set in
  blk_aio_prwv, which is updated here.

2. For all updated functions the parameter type becomes wider so callers
   are safe.

3. In blk_aio_prwv we only store bytes to BlkAioEmAIOCB, which is
   updated here.

4. Other updated functions are wrappers on blk_aio_prwv.

Note that blk_aio_preadv and blk_aio_pwritev become safer: before this
commit, it's theoretically possible to pass qiov with size exceeding
INT_MAX, which than converted to int argument of blk_aio_prwv. Now it's
converted to int64_t which is a lot better. Still add assertions.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211006131718.214235-11-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: tweak assertion and grammar]
Signed-off-by: Eric Blake <eblake@redhat.com>