netdev-dpdk: DPDK v17.11 upgrade

netdev-dpdk: DPDK v17.11 upgrade

This commit adds support for DPDK v17.11:
- minor updates to accomodate DPDK API changes
- update references to DPDK version in Documentation
- update DPDK version in travis' linux-build script
- document DPDK v17.11 virtio driver bug

Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Jan Scheurich <jan.scheurich@ericsson.com>
Tested-by: Jan Scheurich <jan.scheurich@ericsson.com>
Tested-by: Guoshuai Li <ligs@dtdream.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>