vswitch.xml: active-backup and balance-slb bonds must be one-sided.

It doesn't work to hook up an active-backup bond on one switch to an
active-backup bond on another switch, because they might pick different
active interfaces and therefore not pass any traffic.

The same is true of balance-slb because multicast and broadcast
traffic is dropped on ingress to any interface other than the active
interface.

Signed-off-by: Ben Pfaff <blp@nicira.com>