Use virtio-scsi-pci instead of virtio-blk-pci

virtio-scsi supports fstrim, so we don't have to use zerofree.
Use fstrim instead of zerofree to clear unused disk
Unlink before creating a sparse image
Use qemu-img instead of kvm-img for version 1.5
Display which prerequisite that is missing, if any
Check that QEmu is version 1.5 or newer

This is the version where proper support for trim in virtio-scsi was
introduced.
Disable OOM-killer instead of adjusting score
Archive URL is returned correctly as is
Kickstart OpenSuSE 13.1 in a QEmu/KVM virtual machine