virsh: schedinfo --set invalid=value would simply ignore the option

virsh: schedinfo --set invalid=value would simply ignore the option

For example, virsh -c test:///default schedinfo 1 --set P=k would
mistakenly exit successfully, giving no indication that it had failed
to set the scheduling parameter "P".
* tools/virsh.c (cmdSchedinfo): Diagnose an invalid --set j=k option,
rather than silently ignoring it.
* tests/virsh-schedinfo: New test for the above.
* tests/Makefile.am (test_scripts): Add it.
Reported by Jintao Yang in http://bugzilla.redhat.com/586632