i40evf: fix crash when setup Tx queues

i40evf: fix crash when setup Tx queues

This patch fixes the issue:
Testpmd crashed with Segmentation fault when setup tx queues on vf
Steps for reproduce:
  - create one vf device from i40e driver
  - bind vf device to igb_uio and start testpmd

With debugging tools, we saw the struct i40e_vf is cleared after
memcpy(&dev->data->dev_conf, dev_conf, sizeof(dev->data->dev_conf)) in
rte_eth_dev_configure, which should not happen, and the pointer to
i40e_vf isn't in the range of i40e_adapter.

The root cause is the dev_private_size in i40e virtual function driver
struct rte_i40evf_pmd was set incorrectly.

Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Marvin Liu <yong.liu@intel.com>