introduction of volume backing support
tests for configuration parsing of backing param
documentation of backing parameter
new methods for StoragePool in VirtController
new methods for StorageVolume in VirtController
remove now useless attr _virtobj in StoragePool
new methods for Network in VirtController
added methods for Domain in VirtController

Also removed optional boolean parameter overwrite of Domain.create()
since it was always True.

Fixes #22 (fully).