virt.tests.netstress_kill_guest: Workaround udev writing to serial

On some Linux guests such as RHEL 6.2, it's common that when
the succession of rmmods and modprobes happen, udev, SELinux
and other platform software write messages directly to the
serial, confusing aexpect. So let's write some code to
work around this peculiarity, being more robust in handling
these situations.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>

client/virt/tests/netstress_kill_guest.py

@@ -121,17 +121,29 @@ def netload_kill_problem(session_serial):
                 os.kill(pid, signal.SIGCONT)
 
 
+    def send_cmd_safe(session_serial, cmd):
+        logging.debug("Sending command: %s", cmd)
+        session_serial.sendline(cmd)
+        got_prompt = False
+        while not got_prompt:
+            time.sleep(0.2)
+            session_serial.sendline()
+            try:
+                session_serial.read_up_to_prompt()
+                got_prompt = True
+            except aexpect.ExpectTimeoutError:
+                pass
+
+
     def netdriver_kill_problem(session_serial):
         modules = get_ethernet_driver(session_serial)
         logging.debug(modules)
         for _ in range(50):
             for module in modules:
-                session_serial.cmd("rmmod %s" % (module))
-                time.sleep(0.2)
+                send_cmd_safe(session_serial, "rmmod %s" % module)
             for module in modules:
-                logging.debug("Sending command: modprobe %s", module)
-                session_serial.sendline("modprobe %s\n" % (module))
-                time.sleep(0.2)
+                send_cmd_safe(session_serial, "modprobe %s" % module)
+
         kill_and_check(vm)