openflow: Request switch desc on connection

It can be useful to know early on what type of switch you're connected to.
For example, you may need/wish to do special initialization if a switch
supports some extension.  To facilitate this, we now send a switch desc
stats request as part of handshaking by default (it can be disabled
relatively easily), and the result shows up as Connection.description which
should be available by ConnectionUp time.
openflow.nicira: Only enable extensions for OVS

Previously, as long as openflow.nicira was loaded, all switches were
treated as if they supported those extensions.  We now check to see if
the switch actually appears to be OVS.