virt: Print different message if no module found and fix issues

1. When no module found by run_unittest.py, print message:
   "No test module was found!"
2. In run_test() function, move "__import__" and "getattr"
   into try part that, if exception occur in import or getattr,
   print_fail() won't be executed.
3. Fix some issues found by run_pylint.py

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>