The mocked vdisk_list() is based on $source_vdisks, but the real one
only returns images. Add a separate list for other volumes on the
source, e.g. ISO images, which is only considered by the mocked
volume_size_info(). This is in preparation for a test with an ISO
image that is only referenced by a snapshot.
Suggested-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Michal Fox <me@dualfroz.com>