Merge PR #604 (snabbnfv promisc mode) into next

Added the requirement that only one port is defined when promiscuous
mode is being used. This is simply because the best semantics for
other cases is not immediately clear to me and I want to avoid
adopting a dodgy behavior that we will break later.

src/program/snabbnfv/nfvconfig.lua

@@ -31,9 +31,16 @@ function load (file, pciaddr, sockpath)
       local name = port_name(t)
       local NIC = name.."_NIC"
       local Virtio = name.."_Virtio"
+      local vmdq = true
+      if not t.mac_address then
+         if #ports ~= 1 then
+            error("multiple ports defined but promiscuous mode requested for port: "..name)
+         end
+        vmdq = false
+      end
       config.app(c, NIC, require(device_info.driver).driver,
                  {pciaddr = pciaddr,
-                  vmdq = true,
+                  vmdq = vmdq,
                   macaddr = mac_address,
                   vlan = vlan})
       config.app(c, Virtio, VhostUser, {socket_path=sockpath:format(t.port_id)})