Checkbox should support audio devices which don't provide /proc/asound/card#

Some audio devices don't seem to provide a corresponding entry under /proc/asound/card# which results in the following stack trace. So, Checkbox should simply make a best effort and ignore devices which don't provide this information.

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/checkbox/reactor.py", line 89, in fire
    results.append(handler(*args, **kwargs))
  File "/usr/share/checkbox/plugins/registry_info.py", line 31, in gather
    registry_eval_recursive(self._manager.registry, "False")
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 178, in registry_eval_recursive
    values.extend(registry_eval_recursive(value, source, mask))
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 176, in registry_eval_recursive
    for key, value in registry.items():
  File "/usr/lib/python2.6/dist-packages/checkbox/lib/cache.py", line 31, in __call__
    self._cache[(self._instance,) + args] = self._function(self._instance, *args)
  File "/usr/share/checkbox/registries/udev.py", line 516, in items
    return [(d.path, d) for d in devices if not self._ignore_device(d)]
  File "/usr/share/checkbox/registries/udev.py", line 457, in _ignore_device
    if not device.product and device.product_id is None:
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 50, in __getattr__
    return self.get(name)
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 60, in get
    return self.__getitem__(key)
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 119, in __getitem__
    for k, v in self.items():
  File "/usr/share/checkbox/registries/udev.py", line 378, in items
    ("product", self._get_product()),
  File "/usr/share/checkbox/registries/udev.py", line 330, in _get_product
    file = open(path, "r")
IOError: [Errno 2] No such file or directory: '/proc/asound/card1/pcm0p/info'