Run disk format detection also for CD/DVD discs

[skobyda]

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=2215599

[skobyda]

So there is a bug where it's appareant we just automatically set every CD/DVD disc to qcow2.
But a while ago, @jelly added format autodetection at #646 . But I see the autodetection is only limited to Disk image files:

But doesn't run for CD/DVD discs:

My naive approach was to also extend autodetection to CD/DVD discs, expecting to run into some issues. But it seems to work fine.

@jelly you wrote that code. Do you remember if there was any reason why you limited it only for Disk images, and didn't extend it to CD/DVD discs?

[jelly]

@skobyda I can't recall why, but I think it was because I never considered the cd/dvd image option. The option did exist back then, I simply did not take it into account :)

[martinpitt]

Ah, so an ISO file counts as "raw" format -- that's only relevant in the sense of "is it qcow or not"? Then this is straightforward indeed. Thanks!