Add an API to list drivers grouped by device

In the nvidia case with nouveau or fglrx, the backend can know that we have such a device and so present those list of free drivers along the way.

We can imagine a detect.system_device_drivers() call which return a tuple
{'</dev/syspath>': {''nvidia-current': {'virtual': False},
                                   'nouveau': {'virtual': True, 'description': 'Nouveau driver for nvidia cards'},
'</dev/syspath2>": …}

Devices with only external modules will have all the virtual property to False.
Some device like nvidia or ATI which have kernel drivers if no external module is used will have one heuristically detected driver with the driver name and a full description.

Another API we can imagine is:

{'</dev/syspath>': {'model': 'GT218 [GeForce 8400 GS]',
                                   'modalias': 'pci:v000010DEd000010C3sv00sd01bc03sc00i00',
                                   'vendor': 'NVIDIA Corporation'
                                   'drivers': {'nvidia-current': {'free': False,
                                                                                      'from_distro': True
                                                                                      'type': 'package'},
                                                    'nouveau': {'description': 'Nouveau driver for nvidia cards',
                                                                         'type': virtual'},
                                                    }
                                     },
'<…other syspath>':…
}

This has the convenience to somewhat nest the system_driver_packages() call into this system_device_drivers() call.