Make DvrServerRpcCallback a separate callback class

Make DvrServerRpcCallback a separate callback class

RPC has a version of itself. In Neutron a plugin implements
several RPC interface, so a single RPC version doesn't work.
In Mixin callback class approach, RPC versioning depends on
each plugin implementation and it makes harder to maintain
RPC version appropriately. This patch series replaces mixin
RPC callback of server side with a separate class.

This commit handles server-side callback of DVR ML2 RPC interface.

Partial-Bug: #1359416
Change-Id: I1b6383f7b0af5d9aed18eda3a15f21d3504d0347