The performed checks:
* If format is ISO, the file must have correct magic string
* If it's bootable, there must be MBR or GPT
When a check fails on any failable deliverable, it will be logged and
the file removed from metadata (it will still remain on the disk). This
required a change to write the images.json file later (after test
phase).
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
