new class VirtController for all libvirt calls

All calls to libvirt library are now done through the new VirtController
class. There are still few exceptions in StoragePool class though, but
it will be done in next commits. Python libvirt binding is only imported
in lib/VirtController.py however, it will greatly simplify mocking of
Libvirt in unit tests. It is a simple drop/replace change with matching
names for now but it may be improved but more advanced methods will be
added as soon as it could simplify the code in other parts of Cloubed.

Also remove useless Domain.snapshot() method temporarily. Should
reappear one day when properly designed (with #9).

Fixes #22.
domain dependency managed by Domain class

Domain dependency were managed by Domain class at creation time. It may
better be done by global Cloubed class with both coherency checking.

Also renamed:

- Domain.get_disks() -> Domain.get_storage_volumes_names()
- Domain.get_networks() -> Domain.get_networks_names()

And added the following methods:

- Domain.get_networks()
- Domain.get_storage_volumes()
small code simplification

Replace 4 lines by 1 that does basically the same.
remove useless getters in DomainDisk class

Pure cosmetic, no change in logic.
remove useless getters in DomainNetif class

Pure cosmetic, no change in logic.
remove useless getters in DomainTemplate class

Pure cosmetic, no change in logic.
remove useless getters in Domain class

Pure cosmetic, no change in logic.
remove useless getters in DomainEvent class

Pure cosmetic, no change in logic.
remove useless getters in Network class

Pure cosmetic, no change in logic.
remove useless getters in StoragePool class

Pure cosmetic, no change in logic.
remove useless getters in StorageVolume class

Pure cosmetic, no change in logic.
remove useless methods getvirtobj()

In classes Domain, Network and StorageVolume. It is still useful in
StoragePool class because it is currently used in StorageVolume.
remove useless getters in ConfigurationItem class

Pure cosmetic, no change in logic.
remove useless getters in Configuration class

Pure cosmetic, no change in logic.
fixing tests following 1187f2f2
fixing tests following af674073
remove useless getters in ConfigurationLoader class

Pure cosmetic, no change in logic.
fixing tests following 4aab55a8
remove useless getters in ConfigurationStoragePool

Pure cosmetic, no change in logic.
fixing tests following beae2eb5