virDomainBlockStatsFlags was missing a check that was present in
virDomainGetMemoryParameters.  Additionally, I found that the
existing descriptions were a bit hard to read.  A later patch
will fix qemu to return fewer than max parameters if @nparams
was too small on input.

* src/libvirt.c (virDomainGetMemoryParameters)
(virDomainGetBlkioParameters, virDomainGetSchedulerParameters)
(virDomainGetSchedulerParametersFlags):
Tweak documentation wording.
(virDomainBlockStatsFlags): Likewise, and add sanity check.