ova-compose: add raw_image option for disks

[oliverkurth]

This adds the raw_image option to disks. Example:

hardware:
...
    rootdisk:
        type: hard_disk
        parent: sata1
        raw_image: dummy.img

If present, the image file will be converted from raw image format to vmdk format, which will be packaged into the OVA (if configured to create an OVA). If the option disk_image is set, the vmdk file will be named to that value, otherwise, the name will be the same but with any extension stripped and .vmdk appended.

In the above example, the vmdk file will be named dummy.vmdk.

If the vmdk file already exists and is newer than the raw image file it will not be recreated. This is to avoid unnecessary conversions since this is time-intensive. If the raw image file does not exist but the vmdk file does, the vmdk file will be used, with a warning.