common: adding Affinity Groups business entity

Define a new entity in the system called Affinity Group.
Affinity Group will hold a set of VMs (later on could be other entities)
and properties. Each VM can be associated with several groups.
The VM will be scheduled according to its affinity groups
(properties and members) rules.

For more information please refer
to: http://www.ovirt.org/Features/VM-Affinity

Change-Id: I2be28b3a0aede953fcf09e1b6a8f8f99806d9fb5
Bug-Url: https://bugzilla.redhat.com/1036638
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>