pybind: Fix aio_read handling of string buffer

Read data may contain \0, and buf.value interprerts them as string terminator.

Signed-off-by: Mohammad Salehe <salehe+dev@gmail.com>
test: check python rados aio_read with buffers containing null

http://tracker.ceph.com/issues/9547 Refs: #9547

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2545 from dachary/wip-9547-python-rados-truncate

python radio aio_read must not truncate on \000

Reviewed-by: Loic Dachary <loic-201408@dachary.org>