openflow: Implement OF1.4+ OFPMP_QUEUE_DESC multipart message.

openflow: Implement OF1.4+ OFPMP_QUEUE_DESC multipart message.

OpenFlow 1.0 through 1.3 have a message OFPT_QUEUE_GET_CONFIG_REQUEST and
its corresponding reply, for fetching a description of the queues
configured on a given port.  OpenFlow 1.4 changes this message to a
multipart message OFPMP_QUEUE_DESC, which Open vSwitch has not until now
implemented.  This commit adds an implemntation of that message.  Because
the message is a replacement for the former one, this commit implements it
using the same ofp-util functions as the former message, so that the client
code doesn't have to distinguish a difference between versions.

The ovs-ofctl command queue-get-config was previously undocumented (due
only to an oversight).  This commit corrects that and documents the new
feature available with OpenFlow 1.4.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>