mic plugin: Use the "core number" as plugin instance.

mic plugin: Use the "core number" as plugin instance.

Previously, per-core CPU statistics would be dispatched as (e.g.):

  <host>/mic-0/cpu-1-user

This makes graping hard, because it required to parse the type instance to
seperate out different graphs. The core number has therefore been moved to the
plugin instance, so the "mic" plugin behaves more like the "cpu" plugin, e.g.:

  <host>/mic-0-cpu-1/cpu-user