def _expected_device(requested_device):
    prefix = b"/dev/sd"
    if requested_device.startswith(prefix):
        return FilePath(b"/dev").child(b"xvd" + requested_device[len(prefix):])
    raise ValueError(
        "Unsupported requested device {!r}".format(requested_device)
    )
Given a device we requested from AWS EBS, determine the OS device path that
dataset/ETHPy150Open ClusterHQ/flocker/flocker/node/agents/ebs.py/_expected_device