Fix missing bridge for xen instances

Xen instances nic definitions miss the target bridge.

This bug was introduced in commit 503b97a9.

Signed-off-by: Alessandro Cincaglini <alessandro.ciancaglini@gmail.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>