schema: Add new domain elements to support multiple throttle groups

schema: Add new domain elements to support multiple throttle groups

Introduce schema for defining '<throttlegroups>' element which
configures throttling groups which can be configured for multiple
disks.

* Refactor "diskIoTune" to extract common schema "iotune"
* Add new elements '<throttlegroups>'
* <ThrottleGroups> contains <ThrottleGroup> defintion, which references
  "iotune"

Signed-off-by: Chun Feng Wu <danielwuwy@163.com>
Signed-off-by: Harikumar Rajkumar <harirajkumar230@gmail.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>