netdev-dpdk: Add support for virtual DPDK PMDs (vdevs)

netdev-dpdk: Add support for virtual DPDK PMDs (vdevs)

Prior to this commit, the 'dpdk' port type could only be used for
physical DPDK devices. Now, virtual devices (or 'vdevs') are supported.
'vdev' devices are those which use virtual DPDK Poll Mode Drivers eg.
null, pcap. To add a DPDK vdev, a valid 'dpdk-devargs' must be set for
the given dpdk port. The format expected is 'eth_<driver_name><x>' where
'x' is a number between 0 and RTE_MAX_ETHPORTS -1.

For example to add a port that uses the 'null' DPDK PMD driver:

ovs-vsctl set Interface null0 options:dpdk-devargs=eth_null0

Not all DPDK vdevs have been verified to work at this point in time.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Stephen Finucane <stephen@that.guru>  # docs only
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>