vale: permit operation with netmap vale switch.

This patch should enable tcpreplay to inject packets directly to netmap's vale switch.

vale: permit operation with netmap vale switch.  For example:  
    tcpreplay -i vale:tx --netmap my.pcap

This can be tested with pkt-gen as follows:
./pkt-gen -i vale:rx -f rx

Note vale is a learning bridge, so once addresses are learned they are no longer broadcast to all listeners.  Vale is easily modified to bypass learning.