libxl: fix param assignment in domainGetSchedulerParameters
    
    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: Volo M. <vm@vovs.net>