netdev-dpdk: Refactor construct and destruct.

netdev-dpdk: Refactor construct and destruct.

Some refactoring for _construct() and _destruct() methods:
* Rename netdev_dpdk_init() to common_construct(). init() has a
  different meaning in the netdev context.
* Remove DPDK_DEV_ETH and DPDK_DEV_VHOST checks in common_construct()
  and move them to different functions
* Introduce common_destruct().
* Avoid taking 'dev->mutex' in construct and destruct: we're guaranteed
  to be the only thread with access to the object.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>