pybind/mgr/iostat: use correct types

pybind/mgr/iostat: use correct types

for better readability and for the sake of correctness.

* do not cast denominator to float before diving by it
* use '//' for divding an int
* cast int to str as it will be used as an element in a list of str

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 4ffcccf6a4bbded4a240c2bf13020da1d6d6d774)