libvirt.h: consolidate typed parameter handling

libvirt.h: consolidate typed parameter handling

The new type is identical to the three old types that it replaces,
and by creating a common type, this allows future patches to share
common code that manipulates typed parameters.

This change is backwards-compatible in API (recompilation works
without any edits) and ABI (an older client that has not been
recompiled uses the same layout) for code using only public
names; only code using private names (those beginning with _)
will have to adapt.

* include/libvirt/libvirt.h.in (virTypedParameterType)
(VIR_TYPED_PARAM_FIELD_LENGTH, _virTypedParameter): New enum,
macro, and type.
(virSchedParameter, virBlkioParameter, virMemoryParameter):
Rewrite in terms of a common type, while keeping all old public
names for backwards compatibility.
(struct _virSchedParameter, struct _virBlkioParameter)
(struct _virMemoryParameter): Delete - these are private names.
* python/generator.py (enum): Cope with the refactoring.