NSX: fix migration for networks without a subnet

In case the network is without a subnet, calling the validation
logic during the report phase leads to an error because the LSN
would have been already allocated during the migration phase.

Bypass the issue by calling the plugin directly, which is what
the validation logic does in the first place.

Closes-bug: #1313997

Change-Id: I14f77ae3b0cc147c4ea1c79e56bdd809de7c76a0
(cherry picked from commit 47e51e7521784f6a2edcfbf71a9aac0237e76e42)