Changed dangerous default parameter values where used to use None instead.

Changed dangerous default parameter values where used to use None instead.
Renamed variables type and ord since these clash with built-in names.  Renamed
unused variables to mark them as such.  Changed scheduler.py to use *args and
**kwargs rather than the strange non-idiomatic imitation of the same.

Signed-off-by: Ewan Mellor <ewan@xensource.com>