datapath-windows: Move Build*Msg() to Netlink.c

Moving the functions that build netlink messages to Netlink.c from
Vport.c

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
datapath-windows: nuke unused code in Flow.c

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>