Description of problem:
Due to https://bugzilla.redhat.com/show_bug.cgi?id=2179333, any VM connected to a VLAN through a bridge CNI NAD is also connected to the native VLAN 1. This is unintended. We should make sure our users set `"preserveDefaultVlan": false` to prevent this from happening.

Version-Release number of selected component (if applicable):
4.12.5+

How reproducible:
Always

Steps to Reproduce:
1. Create a bridge NAD through the UI

Actual results:
It is created without `"preserveDefaultVlan": false` which causes the issue described above.

Expected results:
The generated NAD contains `"preserveDefaultVlan": false`. Example:
apiVersion: "k8s.cni.cncf.io/v1"
kind: NetworkAttachmentDefinition
metadata:
name: <bridge-network>
annotations:
k8s.v1.cni.cncf.io/resourceName: bridge.network.kubevirt.io/<bridge-interface>
spec:
config: '

{ "cniVersion": "0.3.1", "name": "<bridge-network>", "type": "cnv-bridge", "bridge": "<bridge-interface>", "macspoofchk": true, "vlan": 1, "preserveDefaultVlan": false }

'

Additional info:
This should be changed on all releases starting with 4.12.
The backend bug where this new attribute was introduced: https://bugzilla.redhat.com/show_bug.cgi?id=2179333