Report failure when tap port is not mapped to real dpdk port.

Report failure when tap port is not mapped to real dpdk port.

When using "vdev=" in port section, a tap device and an additional dpdk
port are created. In most cases, we still want to use a real DPDK device
(i.e. bound to an interface such as virtio or a PF or a VF).
PROX will now report by default an error when the used [port] is not a
DPDK bound device.
An additional parameter "virtual=yes" can be added in the port section
in case one want to disable this error checking (e.g. specify additional
parameter for the vdev device).
This patch also fixes a potential endianess issue when configuring IP
addresses.

Change-Id: Iffdd9552308be3b77cfe2067630647acac2c01fe
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>