vconn: Update length of bundled messages.

vconn: Update length of bundled messages.

Variable length messages need their length updated before they can be
added to the bundle.

Message length updating after encoding is sometimes done by the
encoding function, but always latest when the message is sent out.  As
an OpenFlow message is added to a bundle add message, it will not be
sent by itself, and we need to update the length explicitly instead.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>