Clarify the semantic of virDomainGetSchedulerParameters arguments

Clarify the semantic of virDomainGetSchedulerParameters arguments

params and nparams are essential and cannot be NULL. Check this in
libvirt.c and remove redundant checks from the drivers (e.g. xend).

Instead of enforcing that nparams must point to exact same value as
returned by virDomainGetSchedulerType relax this to a lower bound
check. This is what some drivers (e.g. xen hypervisor and esx)
already did. Other drivers (e.g. xend) didn't check nparams at all
and assumed that there is enough space in params.

Unify the behavior in all drivers to a lower bound check and update
nparams to the number of valid values in params on success.