[libvirt] libvirt adds ", boot-on" to qemu-kvm command line - prevents guest boot!

I'm experimenting with libvirt and qemu-kvm using a raw hard disk image that has a bootable DOS partition in it. I noticed that libvirt was adding the ",boot=on" option to the disk section of the qemu-kvm command line. This prevents the guest from booting somehow. If that option is removed, then the guest can boot just fine. What is that option used for and is there any way to cause libvirt to not add it in my scenario? - Bruce Rogers