Bug fix in block_copy tests

Fix some small bugs in configuration file and enhancement test steps for drive_mirroring test

Changes:
1. Change snapshot images file and target image file from /tmp to data/images;
2. Update implement for "bootup with target image" step in drive_mirror.with_powerdown test;
3. Update event check logical in drive mirroring test;
4. Fix some configuration bugs;
5. Update drive_mirror_reboot script, to make it match with test case;
6. Correct test steps and checkpoint in qemu_img.rebase test;