virttest.utils_test: Ignore shell session terminated error when run autotest inside guest

Some stress test inside guest will cause shell session terminated, we
want ignore this kind of problem in some situation. Add a parameter to
support this.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1771 from ypu/ignore_network_break

virttest.utils_test: Ignore network break when run autotest inside guest