common: Introduce new scheduling arch BEs

* PolicyUnit: data stucture representing a single scheduling logic unit.

* ClusterPolicy: container data structure for cluster's policy,
contains attached filters, functions and load balancing logic.

Detailed design: http://wiki.ovirt.org/Features/oVirtSchedulerAPI

Change-Id: I1b6946d51e55b936747c13878d4b710af0267016
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>