This command allows libvirt to attach to an existing QEMU
instance.

 $ qemu-kvm -cdrom ~/demo.iso \
     -monitor unix:/tmp/demo,server,nowait \
     -name foo \
     -uuid cece4f9f-dff0-575d-0e8e-01fe380f12ea  &
 $ QEMUPID=$!
 $ virsh qemu-attach $QEMUPID