Fix race condition in _get_pci_passthrough_devices
Fix race condition in _get_pci_passthrough_devices

The call to _get_pci_passthrough_devices could fail because a
network device could have disappeared which would cause a traceback
in the logs.

This wraps the function in a safe way to return an empty array
if it fails, which will clean-up the logs if the device disappears

Closes-Bug: #1972028
Change-Id: I46d3bbe122d9f8452f168286391bab67ecea3128

openstack/nova,mahak/nova,mahak/nova,mahak/nova,openstack/nova,openstack/nova