Fix missing OVF in OVA image generation

generate-ova.sh script uses a relative path to the OVF template file. This could fail if recipe is run from a different directory.

Fix this by adding the $RECIPEDIR prefix to make the OVF file path absolute.

In the current version of debos, the script has to be run with postprocess: false in order to access the environment variable.

While here, let's make the script to fail if the input disk image doesn't exist.