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
Merge "NSX: fix migration for networks without a subnet"