sched: provide new API shims for remaining drivers

sched: provide new API shims for remaining drivers

Well, the remaining drivers that already had the get/set
scheduler parameter functionality to begin with.

For now, this blindly treats VIR_DOMAIN_SCHEDINFO_CURRENT as
the only supported operation for these 5 domains; it will
take domain-specific patches if more specific behavior is
preferred.

* src/esx/esx_driver.c (esxDomainGetSchedulerParameters)
(esxDomainSetSchedulerParameters): Move guts...
(esxDomainGetSchedulerParametersFlags)
(esxDomainSetSchedulerParametersFlags): ...to new functions.
* src/libxl/libxl_driver.c (libxlDomainGetSchedulerParameters)
(libxlDomainSetSchedulerParameters)
(libxlDomainGetSchedulerParametersFlags)
(libxlDomainSetSchedulerParametersFlags): Likewise.
* src/lxc/lxc_driver.c (lxcGetSchedulerParameters)
(lxcSetSchedulerParameters, lxcGetSchedulerParametersFlags)
(lxcSetSchedulerParametersFlags): Likewise.
* src/test/test_driver.c (testDomainGetSchedulerParams)
(testDomainSetSchedulerParams, testDomainGetSchedulerParamsFlags)
(testDomainSetSchedulerParamsFlags): Likewise.
* src/xen/xen_driver.c (xenUnifiedDomainGetSchedulerParameters)
(xenUnifiedDomainSetSchedulerParameters)
(xenUnifiedDomainGetSchedulerParametersFlags)
(xenUnifiedDomainSetSchedulerParametersFlags): Likewise.