Prevent calling rte_eth_xstats_get not initialized dev

Prevent calling rte_eth_xstats_get not initialized dev

DPDK API forbids calling rte_eth_xstats_get on a port
which has not been setup up yet.

Citing the DPDK docs (reformated):

"
The functions exported by the application Ethernet API to setup a device
designated by its port identifier must be invoked in the following order:

rte_eth_dev_configure()
rte_eth_tx_queue_setup()
rte_eth_rx_queue_setup()
rte_eth_dev_start()
Then, the network application can invoke, in any order, the functions
exported by the Ethernet API to get the MAC address of a given device,
to get the speed and the status of a device physical link, to
receive/transmit [burst of] packets, and so on.
"

original can be found here:
http://dpdk.org/doc/api/rte__ethdev_8h.html#a36ba70a5a6fce2c2c1f774828ba78f8d

Change-Id: I91854b8b0dd12dd028b4b36665cca49f16eac24c
Signed-off-by: Szymon Sliwa <szs@semihalf.com>