tests/qtest/libqos/e1000e: Use IVAR shift definitions

There were still some constants defined in e1000_regs.h.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20221105053010.38037-1-akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>