doc: introduce PVP reference benchmark

doc: introduce PVP reference benchmark

Having reference benchmarks is important in order to obtain
reproducible performance figures.

This patch describes required steps to configure a PVP setup
using testpmd in both host and guest.

Not relying on external vSwitch ease integration in a CI loop by
not being impacted by DPDK API changes.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: John McNamara <john.mcnamara@intel.com>