imagebackend: handle iso images correctly with rbd

iso images are raw from qemu's perspective, so
allow cloning them. They just need to be exposed to
the guest as cdroms instead of disks.

This is handled by the disk config's source_device
attribute, not device_type. Use the correct attribute
in libvirt_info().

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>