ovs_events: Add external ids as metadata

ovs_events: Add external ids as metadata

- Interface meta data are added to dispatched
  values & notification.
  interface metadata:
    - UUID (always);
    - iface-id (if exists)
    - vm-uuid (if exists)
- Fix read callback to poll actual data instead of
  reading cache;
- Code clean up;

Change-Id: I291cb190d31ae091c1d47ce0f5e9d439f6958d61
Signed-off-by: Mytnyk, VolodymyrX <volodymyrx.mytnyk@intel.com>