netdev-dpdk: Refactor dpdk_class_init()

The following changes were made:

- Since we have two dpdk classes, we should split the initial operations needed
  by both classes from the initialization needed by each class.
- The dpdk_ring class does not need an initialization function: it has been
  removed. This also prevents many testcase from failing, because
  dpdk_ring_class_init() was printing an unexpected log message
  (OVS_VSWITCHD_START at tests/ofproto-macros.at:54 check for a specific set of
  startup log messages)
- If the user doesn't pass the --dpdk option we do not register the dpdk*
  classes
- Do not call VLOG_ERR if there are 0 dpdk ethernet device. OVS can now be used
  with dpdk_ring devices.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>