Fixed traceback in EnumerateInstances on LMI_LocalFileSystem.

CIM_ERR_FAILED: Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cmpi_pywbem_bindings.py", line 82, in __call__
    return self.meth(*args, **kwds)
  File "/usr/lib/python2.7/site-packages/cmpi_pywbem_bindings.py", line 487, in enum_instances
    for i in self.proxy.MI_enumInstances(env, op, plist):
  File "/usr/lib/python2.7/site-packages/openlmi/storage/FormatProvider.py", line 144, in enum_instances
    yield self.get_instance(env, model, fmt)
  File "/usr/lib/python2.7/site-packages/openlmi/common/cmpi_logging.py", line 81, in helper_func
    ret = func(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/openlmi/storage/LocalFileSystemProvider.py", line 321, in get_instance
    uuid = self.get_uuid(device, fmt)
UnboundLocalError: local variable 'device' referenced before assignment