Fix extrapolation used in latency measurements

Fix extrapolation used in latency measurements

When doing latency measurements PROX takes into account the
generation or reception of a bulk of packets. For instance, if
PROX receives at time T 4 packets, it knows that the first
packet was received by te NIC before T (the time to receive the other
3 packets, as they were received at maximum link speed).
So the latency data is decreased by the minimum time to receive those
3 packets.
For this PROX was using a default link speed of 10Gbps. This is wrong
for 1Gbps and 40Gbps networks, and was causing for instance issues
on 40 Gbps networks as extrapolating too much, resulting in either
too low latencies or negative numbers (visible as very high latencies).

Change-Id: I4e0f02e8383dd8d168ac50ecae37a05510ad08bc
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>