If we can choose live or config when setting, then we need to
be able to choose which one we are querying.

Also, make the documentation clear that set must use a non-empty
subset (some of the hypervisors fail if params is NULL).

* include/libvirt/libvirt.h.in
(virDomainGetSchedulerParametersFlags): New prototype.
* src/libvirt.c (virDomainGetSchedulerParametersFlags): Implement
it.
* src/libvirt_public.syms: Export it.
* python/generator.py (skip_impl): Don't auto-generate.
* src/driver.h (virDrvDomainGetSchedulerParametersFlags): New
callback.