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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_persistent_boot_device(self):
 """Get the current persistent boot device set for the host."""

        result = self._get_persistent_boot()
        boot_mode = self._check_boot_mode(result)

        if boot_mode == 'bios':
            return result[0]['value']

        value = result[0]['DESCRIPTION']
        if 'HP iLO Virtual USB CD' in value:
            return 'CDROM'

        elif 'NIC' in value or 'PXE' in value:
            return 'NETWORK'

        elif common.isDisk(value):
            return 'HDD'

        else:
            return None