Retrieves the device mounted at mntpoint, or raises
        MountError if none.

def get_dev_at_mountpoint(mntpoint):
        """
        Retrieves the device mounted at mntpoint, or raises
        MountError if none.
        """
        results = util.subp(['findmnt', '-o', 'SOURCE', mntpoint])
        if results.return_code != 0:
            raise MountError('No device mounted at %s' % mntpoint)

        stdout = results.stdout.decode(sys.getdefaultencoding())
        return stdout.replace('SOURCE\n', '').strip().split('\n')[-1]