Mount the device in a temporary place.

def _mount(device):
    '''
    Mount the device in a temporary place.
    '''
    dest = tempfile.mkdtemp()
    res = __states__['mount.mounted'](dest, device=device, fstype='btrfs',
                                      opts='subvol=/', persist=False)
    if not res['result']:
        log.error('Cannot mount device %s in %s', device, dest)
        _umount(dest)
        return None
    return dest