Presence of mac802154_hwsim WPAN network interfaces causes network setup to fail

# Bug report

The presence of a simulated WPAN network interface created using mac802154_hwsim causes an exception:

```
2023-07-17 09:57:00,094 - util.py[WARNING]: failed stage init-local
failed run of stage init-local
------------------------------------------------------------
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 767, in status_wrapper
ret = functor(name, args)
^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 433, in main_init
init.apply_network_config(bring_up=bring_up_interfaces)
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 887, in apply_network_config
netcfg, src = self._find_networking_config()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 828, in _find_networking_config
if self.datasource and hasattr(self.datasource, "network_config"):
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/sources/DataSourceConfigDrive.py", line 176, in network_config
self._network_config = openstack.convert_net_json(
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/sources/helpers/openstack.py", line 717, in convert_net_json
known_macs = net.get_interfaces_by_mac()
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 926, in get_interfaces_by_mac
return get_interfaces_by_mac_on_linux(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 1004, in get_interfaces_by_mac_on_linux
for name, mac, driver, _devid in get_interfaces(
^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 1120, in get_interfaces
ret.append((name, mac, driver, device_devid(name)))
^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 367, in device_devid
dev_id = read_sys_net_safe(devname, "device/device")
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 107, in read_sys_net_safe
return read_sys_net(
^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 76, in read_sys_net
contents = util.load_file(dev_path)
^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 1531, in load_file
with open(fname, "rb") as ifh:
^^^^^^^^^^^^^^^^^
IsADirectoryError: [Errno 21] Is a directory: '/sys/class/net/wpan1/device/device'
```

`/sys/class/net/wpan1/device` is a symlink to the directory `/sys/devices/platform/mac802154_hwsim/ieee802154/phy1`, and `/sys/class/net/wpan1/device/device` is a symlink to the directory `/sys/devices/platform/mac802154_hwsim`.

Perhaps `read_sys_net()` can handle EISDIR?

## Steps to reproduce the problem

1. Set up a WPAN network interface using mac802154_hwsim
2. Run cloud-init in such a way that it needs to configure networking

## Environment details
- Cloud-init version: 22.4.2
- Operating System Distribution: Debian
- Cloud provider, platform or installer type: OpenStack

## cloud-init logs

[cloud-init.tar.gz](https://github.com/canonical/cloud-init/files/12075991/cloud-init.tar.gz)