Change to use virtio_blk (virtio block device) by default.

Change to use virtio_blk (virtio block device) by default.

virtio_blk is the fast, virt-native block device driver
supported by qemu and KVM.  Note that virtio_blk device
names are called /dev/vd*.

Existing scripts should continue working because device name
translation will silently change device names of the form
/dev/sd* to /dev/vd* as required.

See also:
http://libguestfs.org/guestfs.3.html#block_device_naming