Unk interface gets invalid literal for int()

[Josh VanDeraa]

Environment

• Python version: 3.8
• napalm version: 3.3.1
• napalm-panos version: 0.5.4

Expected Behavior

Expected get_interfaces() to work

Observed Behavior

In [45]: device.get_interfaces()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-45-89ef61d8d3bf> in <module>
----> 1 device.get_interfaces()

~/.cache/pypoetry/virtualenvs/sandbox-python-Fd9X2G4n-py3.8/lib/python3.8/site-packages/napalm_panos/panos.py in get_interfaces(self)
    566                 interface["speed"] = 0
    567             else:
--> 568                 interface["speed"] = int(interface["speed"])
    569             interface["mac_address"] = standardize_mac(interface_info.get("mac"))
    570             interface["description"] = "N/A"

Steps to Reproduce

1. Connect NAPALM-PANOS to a VM PANOS device
2. Execute the get_interfaces() getter
3. See the error