DPDK: Fix possible crash when TXing to agent.
DPDK: Fix possible crash when TXing to agent.

If anything calls agent's vif->vif_tx(), it will call
agent_tx(). In DPDK mode agent_rx() calls dpdk_if_tx(),
where TX to a queue pointed by vr_dpdk.packet_ring occurs.
If packet ring has not been created yet, there is NULL
pointer dereference and crash.

This change makes sure that vif_tx() poins to vif_discard_tx()
as long as packet ring is not created. Then, after ring is
created successfully, vif_tx() is changed to agent_tx().

Closes-bug: #1513213
Change-Id: I2bf3b28ee26da700de83fda4a1d295206208070c

codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter