doc: rework section on virtio-user as exception path

doc: rework section on virtio-user as exception path

This patch extensively reworks the howto guide on using virtio-user for
exception packets. Changes include:

* rename "exceptional path" to "exception path"
* remove references to uio and just reference vfio-pci
* simplify testpmd command-lines, giving a basic usage example first
  before adding on detail about checksum or TSO parameters
* give a complete working example showing traffic flowing through the
  whole system from a testpmd loopback using the created TAP netdev
* replace use of "ifconfig" with Linux standard "ip" command
* other general rewording.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>