First of all - which Qubes version? I run Qubes 3.2. Where is the ISO you want to attach - in dom0, or in some other VM? I put the ISO in another VM, sys-net.
I took a look at /var/log/libvirt/libxl/libxl-driver.log for some details. The log looked like this: https://imgur.com/a/rK9Bq -- You received this message because you are subscribed to the Google Groups "qubes-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to qubes-users+unsubscr...@googlegroups.com. To post to this group, send email to qubes-users@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/qubes-users/87dd2f00-4da4-4dcc-926b-c20737125793%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.