qemu.tests: new tests for checking qemu-img function

New tests to check convert/commit/rebase qcow2 format image/snapshot via qemu-img tool;
'md5sum' used to check image content not changed before/after  convert/commit/rebase a qcow2 format image; For windows guest, 'md5sum' tool can download form http://www.brothersoft.com/md5sum-81781.html and md5sum install patch need to configured in share/cfg/guest-os/Windows.cfg;

Thanks,
Xu