Make bytes argument int64_t to be consistent with modern block-layer.
Callers should be OK with it as type becomes wider.

What is inside functions?

- Conversion from int64_t to size_t. Still, we
can't have a buffer larger than SIZE_MAX, therefore bytes should not be
larger than SIZE_MAX as well. Add an assertion.

- Passing to blk_co_pwritev() / blk_co_preadv() which already has
  int64_t bytes argument.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211007175243.642516-2-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: spelling fix]
Signed-off-by: Eric Blake <eblake@redhat.com>