virttest.standalone_test:avoid to division by zero

When run the command ./run -t qemu as default,
all the case may skip. and this will raise an error:
TESTS: 13
(1/13) migrate.default.tcp: SKIP
(2/13) migrate.default.unix: SKIP
(3/13) migrate.default.exec: SKIP
(4/13) migrate.default.fd: SKIP
(5/13) migrate.default.mig_cancel: SKIP
(6/13) migrate.with_set_speed.tcp: SKIP
(7/13) migrate.with_set_speed.unix: SKIP
(8/13) migrate.with_set_speed.exec: SKIP
(9/13) migrate.with_set_speed.fd: SKIP
(10/13) migrate.with_reboot.tcp: SKIP
(11/13) migrate.with_reboot.unix: SKIP
(12/13) migrate.with_reboot.exec: SKIP
(13/13) migrate.with_reboot.fd: SKIP
TOTAL TIME: 9.58 s
Internal error, traceback follows...
Traceback (most recent call last):
  File "/home/Mike/autotest/client/tests/virt/virttest/standalone_test.py", line 872, in run_tests
    _job_report(job_elapsed_time, n_tests, n_tests_skipped, n_tests_failed)
  File "/home/Mike/autotest/client/tests/virt/virttest/standalone_test.py", line 659, in _job_report
    float(n_tests - n_tests_skipped)) * 100)
ZeroDivisionError: float division by zero

So just check if n_tests equals to n_tests_skipped to avoid
this error

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>