support copy-on-write backing image

- convert ovf vmdk to qcow2 image to support this
- remove vmdk when conert succeeded.
- backing image will be created with qemu-img create -b

- TODO: check backing image when removing box definition.

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
fix qemu-img error when 2nd time in OVF.

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
Merge pull request #63 from miurahr/use_backing_image

support copy-on-write backing image