virt.qemu.enospc:Fix the image_name change errors

Currently, the image_name has changed, the old fashion is like:
/tmp/kvm_autotest_root/images/f18-64, but the new is
images/f18-64.

In test, need to use data_dir to get the images dir.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>