netdev-dpdk: Deprecate ring ports.

netdev-dpdk: Deprecate ring ports.

'dpdkr' a.k.a. DPDK ring ports has really poor support in OVS and not
tested on a regular basis.  These ports are intended to work via
shared memory with another DPDK secondary process, but there are lots
of limitations for using this functionality in practice.  Most of them
connected with running secondary DPDK application and memory layout
issues.  More details are available in DPDK guide:
https://doc.dpdk.org/guides-18.11/prog_guide/multi_proc_support.html#multi-process-limitations

Beside the functional limitations it's also hard to use this
functionality correctly.  User must be sure that OVS and secondary DPDK
application are running on different CPU cores, which is hard because
non-PMD threads could float over available CPU cores.  This or any
other misconfiguration will likely lead to crash of OVS.

Another problem is that the user must actually build the secondary
application with the same version of DPDK that was used for OVS build.

Above issues are same as we have while using DPDK pdump.

Beside that, current implementation in OVS is not able to free
allocated rings that could lead to memory exhausting.

Initially these ports was added to use with IVSHMEM for a fast
zero-copy HOST<-->VM communication.  However, IVSHMEM is not used
anymore.  IVSHMEM support was removed from DPDK in 16.11 release
(instructions for IVSHMEM were removed from the OVS docs almost 3 years
ago by commit 90ca71dd317f ("doc: Remove ivshmem instructions.")) and
the patch for QEMU for using regular files as a device backend is no
longer available.  That makes DPDK ring ports barely useful in real
virtualization environment.

This patch adds a deprecation warnings for run-time port creation
and documentation.  Claiming to completely remove this functionality
from OVS in one of the next releases.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Ian Stokes <ian.stokes@intel.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>