python: events: Fix C->Python handle callback prototype

python: events: Fix C->Python handle callback prototype

If registering our own event loop implementation written in python,
any handles or timeouts callbacks registered by libvirt C code must
be wrapped in a python function. There is some argument trickery that
makes this all work, by wrapping the user passed opaque value in
a tuple, along with the callback function.

Problem is, the current setup requires the user's event loop to know
about this trickery, rather than just treating the opaque value
as truly opaque.

Fix this in a backwards compatible manner, and adjust the example
python event loop to do things the proper way.