[libvirt-users] sco 5.0.5 not booting on hard disk

Hello, I am trying to run SCO unix 5.0.5 with KVM. I have created the Virtual Machine with virt-manager. The installation runs correctly, but when the virtual machine reboots from hard disk, i obtain this message : "FATAL: No bootable device." If i run manually the virtual machine without the suboption "boot=on" in the "-drive" option, the virtual machine boots correctly. Why is "boot=on" preventing my VM to boot ? Is there a way to modify the .xml file for this VM to inform libvirt to not add "boot=on" ? Thanks Jean Michault.