Bug fix for iperf test

[vaishnavibhat]

• Removed problematic tearDown code: Deleted the unconditional restore_slave_cfg_file() that was causing the FileNotFoundError in the task description
• Added exception handling for is_vnic() check: Wrapped the is_vnic() call in a try-except block to gracefully handle cases where the sysfs file doesn't exist
• Used local variable is_vnic: Replaced direct calls to self.networkinterface.is_vnic() with a local boolean variable that defaults to False if the check fails
• Added default value for iperf_pthread: Set to 1 to ensure it's always defined, even when speed is not 100000 or 25000
• Fix pycodestyle

[vaishnavibhat]

Before the patch:

avocado run iperf_test.py -m iperf_test.py.data/iperf_virt_test.yaml --max-parallel-tasks=1

JOB ID : 3dec572a82ac243abfeac63aeadf2105624f6bfb
JOB LOG : /home/OpTest/avocado-fvt-wrapper/results/job-2026-06-15T11.53-3dec572/job.log
(1/2) iperf_test.py:Iperf.test;run-mtu-1500-1a1a: STARTED
(1/2) iperf_test.py:Iperf.test;run-mtu-1500-1a1a: ERROR: Could not restore the config file [Errno 2] No such file or directory: '/etc/NetworkManager/system-connections/enP16389p1s0.nmconnection' (186.28 s)

After patch: