schema: Add new domain elements to support multiple throttle filters

schema: Add new domain elements to support multiple throttle filters

Introduce schema for defining '<throttlefilters>' element which
references throttling groups to form filter chain in qemu for specific
disk

* Add new elements '<throttlefilters>'
* <ThrottleFilters> can include multiple throttlegroup references to
  form filter chain in qemu
* Chained throttle filters feature in qemu is described at
  https://gitlab.com/qemu-project/qemu/blob/master/docs/throttle.txt

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>