Due to a copy and paste error, the scheduler 'cap' parameter
was over-writing the 'weight' parameter when preparing the
return parameters in libxlDomainGetSchedulerParametersFlags.
As a result, the scheduler weight was never shown when getting
schedinfo and setting the weight failed as well

virsh  schedinfo testvm
Scheduler      : credit
cap            : 0

virsh  schedinfo testvm --cap 50 --weight 500
Scheduler      : credit
error: invalid scheduler option: weight

The obvious fix is to assign the 'caps' parameter to the correct
item in the parameter list.
Reported-by: NVolo M. <vm@vovs.net>