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

1. Add 'timeout' parameter to join() function of BackgroundTest class in virttest/utils_test.py, set it to 600s defaultly.
2. Instead time.sleep, use join() to control timeout in qemu/tests/migration.py

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>