Unload a kext by forking into kextunload.

def LegacyKextunload(self, cf_bundle_identifier):
    """Unload a kext by forking into kextunload."""
    error_code = OS_SUCCESS
    bundle_identifier = self.CFStringToPystring(cf_bundle_identifier)
    try:
      subprocess.check_call(['/sbin/kextunload', '-b', bundle_identifier])
    except subprocess.CalledProcessError as cpe:
      logging.debug('failed to unload %s:%s', bundle_identifier, str(cpe))
      error_code = -1
    return error_code