Thank you for using Ubuntu and taking the time to report a bug. For now, you must add the readonly file to /etc/apparmor.d/libvirt/libvirt-<uuid> (don't add it to the dynamically generated libvirt-<uuid>.files file). After that you can shutdown the virtual machine and after that it will work as expected.
I am going to mark this wishlist for now. While this is a bug, libvirt does not support snapshotting by using cow files. This will be fixed when the upstream sVirt plugin framework supports snapshotting via cow files.
Thank you for using Ubuntu and taking the time to report a bug. For now, you must add the readonly file to /etc/apparmor. d/libvirt/ libvirt- <uuid> (don't add it to the dynamically generated libvirt- <uuid>. files file). After that you can shutdown the virtual machine and after that it will work as expected.
I am going to mark this wishlist for now. While this is a bug, libvirt does not support snapshotting by using cow files. This will be fixed when the upstream sVirt plugin framework supports snapshotting via cow files.