ovs-ofctl: Use vconn as a parameter of dump_stats_transaction()

ovs-ofctl: Use vconn as a parameter of dump_stats_transaction()

In order to form a stats message for the prevailing OpenFlow version
of a vconn the vconn needs to be open at the time the request is
encoded. Thus there is no longer a case where it makes sense to
use dump_stats_transaction() without a vconn already being open and
available to pass as a parameter.

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