vonn: Allow snoops to use the OpenFlow version of the controller connection

vonn: Allow snoops to use the OpenFlow version of the controller connection

Override the allowed versions of the snoop vconn so that
only the version of the controller connection is allowed.
This is because the snoop will see the same messages as the
controller.

Without this change the snoop will try to negotiate a connection
using the default allowed versions, or in other words only
allow OpenFlow 1.0. This breaks snoops for controller connections
using other OpenFlow versions, that is OpenFlow 1.2.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>