Changes

Support detecting installer images on normal disks

Many guides for using QEMU just attach the ISO as an ordinary disk to
the VM, to avoid messing about with virtio-scsi or USB. These don't
show up for cd, maybe-usb-floppy or usb-partition, only disk, so we
don't currently detect them, prompting the user to load drivers, even
though they know their disk is there and visible as a proper block
device already. Add list-devices disk as an additional set of devices
to search after everything else to support this use case.

Closes: #851429

Add changelog