The syntax for the _CHECK variables and the contents of the PARAMETERS
  dict is the same, see the docstring for BaseHypervisor.PARAMETERS.
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        _FILE_CHECK = utils.IsNormAbsPath, "must be an absolute normal... 
      stub for file checks, without the required flag
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        _DIR_CHECK = utils.IsNormAbsPath, "must be an absolute normali... 
      stub for directory checks, without the required flag
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        _CPU_MASK_CHECK = _IsCpuMaskWellFormed, "CPU mask definition i...
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        REQ_FILE_CHECK = True,+ _FILE_CHECK 
      mandatory file parameter
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        OPT_FILE_CHECK = False,+ _FILE_CHECK 
      optional file parameter
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        REQ_DIR_CHECK = True,+ _DIR_CHECK 
      mandatory directory parametr
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        OPT_DIR_CHECK = False,+ _DIR_CHECK 
      optional directory parameter
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        NET_PORT_CHECK = True, lambda x: x > 0 and x < 65535, "invalid...
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        OPT_CPU_MASK_CHECK = False,+ _CPU_MASK_CHECK
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        REQ_CPU_MASK_CHECK = True,+ _CPU_MASK_CHECK
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        NO_CHECK = False, None, None, None, None 
      parameter without any checks at all
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        REQUIRED_CHECK = True, None, None, None, None 
      parameter required to exist (and non-false), but without other 
      checks; beware that this can't be used for boolean parameters, where 
      you should use NO_CHECK or a custom checker
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        MIGRATION_MODE_CHECK = True, lambda x: x in constants.HT_MIGRA...
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