Document the parameter names that will be used by
virDomain{Get,Set}SchedulerParameters{,Flags}, rather than
hard-coding those names in each driver, to match what is
done with memory, blkio, and blockstats parameters.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_SCHEDULER_CPU_SHARES)
(VIR_DOMAIN_SCHEDULER_VCPU_PERIOD)
(VIR_DOMAIN_SCHEDULER_VCPU_QUOTA, VIR_DOMAIN_SCHEDULER_WEIGHT)
(VIR_DOMAIN_SCHEDULER_CAP, VIR_DOMAIN_SCHEDULER_RESERVATION)
(VIR_DOMAIN_SCHEDULER_LIMIT, VIR_DOMAIN_SCHEDULER_SHARES): New
field name macros.
* src/qemu/qemu_driver.c (qemuSetSchedulerParametersFlags)
(qemuGetSchedulerParametersFlags): Use new defines.
* src/test/test_driver.c (testDomainGetSchedulerParamsFlags)
(testDomainSetSchedulerParamsFlags): Likewise.
* src/xen/xen_hypervisor.c (xenHypervisorGetSchedulerParameters)
(xenHypervisorSetSchedulerParameters): Likewise.
* src/xen/xend_internal.c (xenDaemonGetSchedulerParameters)
(xenDaemonSetSchedulerParameters): Likewise.
* src/lxc/lxc_driver.c (lxcSetSchedulerParametersFlags)
(lxcGetSchedulerParametersFlags): Likewise.
* src/esx/esx_driver.c (esxDomainGetSchedulerParametersFlags)
(esxDomainSetSchedulerParametersFlags): Likewise.
* src/libxl/libxl_driver.c (libxlDomainGetSchedulerParametersFlags)
(libxlDomainSetSchedulerParametersFlags): Likewise.