Enchance DMI to include DataValueType in Events and also clarification for Transceiver Events

The following enhancements are needed in the Device Management Interface for proper support for Transceiver related Events:

1. As of today the threshold values and the observed value is of type ValueType . So for values  which could be expressed in more than one type of unit; for example voltage threshold can be in VOLTS_AC or VOLTS_DC the user would not know the actual type of the value. Proposal is to include a DataValueType along with the ValueType
2. Introduce another enum for measuring power in VALUE_TYPE_DBM DataValueType 
3. Clarify using comments that the threshold based events on the transceivers should really be configured on the ports components of that transceiver and not on the transceiver component itself. This is because there could be different thresholds on the different ports of a transceiver (example for a COMBO transceiver the power thresholds could be different for the GPON vs XGSPON)