IVirtualSystemDescription tests

Hi!,

This is a unittest for the new IVirtualSystemDescription class described in #15. I've changed the values accepted by the set_final_value method so you now can pass:
  
  - Instances of Enum subclasses like AudioControllerType, for the devices.
  - Instances of basestring, for machine name (and maybe others).
  - Instances of int, for values like CPU count, and the memory size.

I've added a minimalist .ova file as a fixture.

The current test cover the change of this values:
  - Machine name
  - CPU
  - Memory
  - Sound card.

I've also added the skeleton for the following values:
  - Usb controller
  - Network adapter
  - Cdrom
  - Hard disk controller (IDE, SAS, SATA, SCSI).

Some code is needed for the "enable/disable" feature.

Best regards,
Roberto