sched: introduce virDomainGetSchedulerParametersFlags

sched: introduce virDomainGetSchedulerParametersFlags

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.