Do not add layer2 interfaces to a virtual router

Description

Prevent layer2 interfaces being added to virtual routers

Motivation and Context

The origin of this PR stems downstream from pan-os-ansible, where adding a aggregate interface with mode set to layer2 results in an error. This error seems to be due to the network module trying to add the layer2 interface to a virtual-router.

code example

- panos_aggregate_interface:
    if_name: "ae2"
    mode: layer2
    lacp_enable: True
    lldp_enabled: True
    lldp_profile: default
    commit: False

As layer2 interfaces do not have any routable properties, and cannot be added to a virtual-router in the PAN-OS interface, they should be excluded.

downstream error message

The full traceback is:
  File "/tmp/ansible_panos_aggregate_interface_payload_8n5bzkkx/ansible_panos_aggregate_interface_payload.zip/ansible_collections/paloaltonetworks/panos/plugins/modules/panos_aggregate_interface.py", line 297, in main
  File "/home/mvandam/.local/lib/python3.10/site-packages/panos/network.py", line 433, in set_virtual_router
    return self._set_reference(
  File "/home/mvandam/.local/lib/python3.10/site-packages/panos/base.py", line 1664, in _set_reference
    obj.update(reference_var)
  File "/home/mvandam/.local/lib/python3.10/site-packages/panos/base.py", line 712, in update
    device.xapi.edit(
  File "/home/mvandam/.local/lib/python3.10/site-packages/panos/base.py", line 3899, in method
    raise the_exception
fatal: [fra01.lsvpn-hub.leaseweb.org]: FAILED! => {
    "changed": false,
    "invocation": {
        "module_args": {
            "adjust_tcp_mss": null,
            "api_key": null,
            "comment": null,
            "commit": false,
            "create_dhcp_default_route": null,
            "dhcp_default_route_metric": null,
            "enable_dhcp": null,
            "if_name": "ae2",
            "ip": null,
            "ip_address": null,
            "ipv4_mss_adjust": null,
            "ipv6_enabled": null,
            "ipv6_mss_adjust": null,
            "lacp_enable": true,
            "lacp_mode": null,
            "lacp_passive_pre_negotiation": null,
            "lacp_rate": null,
            "lldp_enabled": true,
            "lldp_profile": "default",
            "management_profile": null,
            "mode": "layer2",
            "mtu": null,
            "netflow_profile": null,
            "password": null,
            "port": 443,
            "provider": {},
            "state": "present",
            "template": null,
            "username": "admin",
            "vr_name": "default",
            "vsys": null,
            "zone_name": null
        }
    },
    "msg": "Failed setref:  interface 'ae2' is not a valid reference"
}

How Has This Been Tested?

I have tested and can confirm that layer2 and layer3 aggregate interfaces are now correctly deployed. This has been tested against PAN-OS 10.1.6

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist

[ ] I have updated the documentation accordingly.
[X] I have read the CONTRIBUTING document.
[ ] I have added tests to cover my changes if appropriate.
[ ] All new and existing tests passed.

PaloAltoNetworks / pan-os-python