So that the man page will display:

  The permitted values are: NM_SETTING_IP6_CONFIG_ADDR_GEN_MODE_EUI64
  (0) or NM_SETTING_IP6_CONFIG_ADDR_GEN_MODE_STABLE_PRIVACY (1).

instead of

  The permitted values are: "eui64" or "stable-privacy".

since the latter is not useful at all for a int32 property.
Unfortunately the enum names are quite long and don't look very well
in a table, but that's another problem.