tests/avocado: re-factor igb test to avoid timeouts

The core of the test was utilising "ethtool -t eth1 offline" to run
through a test sequence. For reasons unknown the test hangs under some
configurations of the build on centos8-stream. Fundamentally running
the old fedora-31 cloud-init is just too much for something that is
directed at testing one device. So we:

  - replace fedora with a custom kernel + buildroot rootfs
  - rename the test from IGB to NetDevEthtool
  - re-factor the common code, add (currently skipped) tests for other
     devices which support ethtool
  - remove the KVM limitation as its fast enough to run in KVM or TCG

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230322145529.4079753-1-alex.bennee@linaro.org>