dpdk: Decoupling the meaning of xd->device_index in dpdk_plugin

Prior to the change, dpdk plugin assumes xd->device_index is
used both as index for internal dpdk_main->devices array
and DPDK port index to call into DPDK APIs.

However, when running on top of Failsafe PMDs,
DPDK port index range may no longer be contiguous (as noted:
http://dpdk.org/ml/archives/dev/2018-March/092375.html
for related changes in DPDK). Because this, dpdk plugin can
no longer iterate through all available DPDK ports
with a for 0->rte_eth_dev_count() loop and the assumption of
device_index no longer holds.

This is part of initial effort to enable vpp running over
dpdk on failsafe PMD in Microsoft Azure(3/4).

Change-Id: I416fd80f2d40e12e139f8f3492814da98343eae7
Signed-off-by: Rui Cai <rucai@microsoft.com>