vconn: Fix parameters for vconn_open(), vconn_open_block(), pvconn_open().

vconn: Fix parameters for vconn_open(), vconn_open_block(), pvconn_open().

The customary parameter order in Open vSwitch is to put input parameters
before output parameters, but vconn_open() and pvconn_open() had the 'dscp'
input parameter at the end, which bugged me a bit.  Also,
vconn_open_block() didn't take a 'dscp' parameter at all even though it's
otherwise a wrapper around vconn_open().  This commit fixes all that up.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>