af_packet: add PMD for AF_PACKET-based virtual devices

af_packet: add PMD for AF_PACKET-based virtual devices

This is a Linux-specific virtual PMD driver backed by an AF_PACKET
socket.  This implementation uses mmap'ed ring buffers to limit copying
and user/kernel transitions.  The PACKET_FANOUT_HASH behavior of
AF_PACKET is used for frame reception.  In the current implementation,
Tx and Rx queues are always paired, and therefore are always equal
in number -- changing this would be a Simple Matter Of Programming.

Interfaces of this type are created with a command line option like
"--vdev=eth_af_packet0,iface=...".  There are a number of options available
as arguments:

 - Interface is chosen by "iface" (required)
 - Number of queue pairs set by "qpairs" (optional, default: 1)
 - AF_PACKET MMAP block size set by "blocksz" (optional, default: 4096)
 - AF_PACKET MMAP frame size set by "framesz" (optional, default: 2048)
 - AF_PACKET MMAP frame count set by "framecnt" (optional, default: 512)

Signed-off-by: John W. Linville <linville@tuxdriver.com>
[Thomas: disable because of incompatibility with some kernels]