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>
(cherry picked from commit 8bda44ff37fd04a0fc9498fbbc22f0daf515d721)
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>
(cherry picked from commit 226c0c7ac6ee95ff2c1665d4e7164e2962c0346e)
Merge pull request #2548 from dachary/wip-9547-python-rados-truncate-firefly

python radio aio_read must not truncate on \000 (firefly)

Reviewed-by: Samuel Just <sam.just@inktank.com>