libvirt, vSRX3.0 (19.2R1), file.iso attached as ide cdrom and contains only a single file, juniper.conf (in the root of the iso img)
According to the docs this should be mounted and load overrided as the config on boot.
I see no indication of an attempt to mount it in the log messages. I can manually mount the cdrom once the vsrx has booted. I can even load override from the file on the manually mounted cdrom/iso file system. However the config is not loaded of the iso mount as best I can tell.