fix(sys-kernel/coreos-kernel): Enable virtio block device support.

Enable virtual block devices to support qemu drives configured with
if=virtio instead of the default emulated hardware interface. While I'm
at it throw in the virtio scsi module just in case a need for fancier
devices comes along at some point.
Merge pull request #123 from marineam/virtio

fix(sys-kernel/coreos-kernel): Enable virtio block device support.