express: Correct a check
express: Correct a check

We were passing the unattended disk path to created domains even when
unattended installation was being disabled and disk image wasn't being
copied/created so unattended installs were ending-up with this fatal
error:

qemu-kvm: -drive file=/home/user/.cache/gnome-boxes/fedora14-unattended.img,if=none,id=drive-scsi0-0-1,format=raw: could not open disk image /home/user/.cache/gnome-boxes/fedora14-unattended.img: No such file or directory

vbenes/gnome-boxes,vbenes/gnome-boxes