virt: Add 'timeout' parameter to join() function of BackgroundTest class

Add 'timeout' parameter to join() function of BackgroundTest class in
virttest/utils_test.py, set default value to 600s.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #829 from liyangfnst/threading

virt: Add 'timeout' parameter to join() function of BackgroundTest class