pybind/cephfs: Fix setxattr function. Pass value as character pointer

This passes the value of the xattr correctly and makes it work.
python-rados: refactor class Rados a little bit

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Merge pull request #3171 from tchaikov/refactor-rados-class

python-rados: refactor class Rados a little bit

Reviewed-by: Josh During <jdurgin@redhat.com>
Reviewed-by: Loic Dachary <ldachary@redhat.com>