core/boot: Use modern Python names

Some of the code in here used old Python attribute names like im_class
which have been outdated for quite a while.  Hopefully the new ones work
on PyPy and Jython and whatnot also.
openflow: Bytes vs. strings fixups

Fixed a few lurking places where we were using strings instead of bytes.
openflow: Remove useless import
libopenflow: Use integer division