make resize volume backed aware

make resize volume backed aware.
main changes are as follows:
1. check volume backed or not (is_volume_backed)
2. if volume backed, then no longer needs ssh and copy image, qemu-img. so do not execute when volume backed
3. after done resize, if different original cnode and target cnode, then delete vm config directory (/var/lib/nova/instances/<vm-id>)

please review and merge.
thanks.