tests: Add error.context() for nicdriver_unload.py

Log test steps to logging.info.
Rename '_' to i.
Signed-off-by: NFeng Yang <fyang@redhat.com>
Acked-by: NShuping Cui <scui@redhat.com>

tests/nicdriver_unload.py

 import logging, os, time
 from autotest.client import utils
+from autotest.client.shared import error
 from virttest import utils_test, utils_net
 
 
+@error.context_aware
 def run_nicdriver_unload(test, params, env):
     """
-    Test nic driver.
+    Test nic driver load/unload.
 
     1) Boot a VM.
     2) Get the NIC driver name.
-    3) Repeatedly unload/load NIC driver.
-    4) Multi-session TCP transfer on test interface.
+    3) Multi-session TCP transfer on test interface.
+    4) Repeatedly unload/load NIC driver during file transfer.
     5) Check whether the test interface should still work.
 
     @param test: QEMU test object.
     @param params: Dictionary with the test parameters.
     @param env: Dictionary with test environment.
     """
+
+
     timeout = int(params.get("login_timeout", 360))
     vm = env.get_vm(params["main_vm"])
     vm.verify_alive()
     session_serial = vm.wait_for_serial_login(timeout=timeout)
 
+    error.context("Get NIC interface name in guest.", logging.info)
     ethname = utils_net.get_linux_ifname(session_serial,
                                                vm.get_mac_address(0))
 
@@ -39,18 +44,25 @@ def run_nicdriver_unload(test, params, env):
 
     try:
         threads = []
-        for _ in range(int(params.get("sessions_num", "10"))):
+        for i in range(int(params.get("sessions_num", "10"))):
+            txt = "File transfer on test interface. Thread %s" % i
+            error.context(txt, logging.info)
             thread = utils.InterruptedThread(utils_test.run_file_transfer,
                                              (test, params, env))
             thread.start()
             threads.append(thread)
 
         time.sleep(10)
+        logging.info("Repeatedly unload/load NIC driver during file transfer.")
         while threads[0].isAlive():
             session_serial.cmd("sleep 10")
+            error.context("Shutdown the driver for NIC interface.", logging.info)
             session_serial.cmd("ifconfig %s down" % ethname)
+            error.context("Unload  NIC driver.", logging.info)
             session_serial.cmd("modprobe -r %s" % driver)
+            error.context("Load NIC driver.", logging.info)
             session_serial.cmd("modprobe %s" % driver)
+            error.context("Activate NIC driver.", logging.info)
             session_serial.cmd("ifconfig %s up" % ethname)
     except Exception:
         for thread in threads: