kvm: remove classes that nothing references any more

  - IvsVifDriver: VIF driver for Big Switch IVS. It could only be
    activated by naming it in libvirt.vif.driver in agent.properties,
    and the Big Switch plugin itself never refers to it.
  - ManagedNfsStorageAdaptor: picked up by the storage adaptor
    reflection scan, but the management server never creates a pool of
    type ManagedNFS, so the adaptor was never selected. The enum value
    is left in place.
  - LibvirtNetworkDef: builder for libvirt <network> XML, never used.
  - KVMGuestOsMapper, KVMStorageResource, StorageAdaptorInfo,
    KVMVirtualDisk: no references anywhere in the tree.

Author: wido