doc: Changed libvirt example to use virtio.

fixes: #4469
doc: Moved ceph osd create to first step.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added info on OSD naming, and example.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added additional note on first step of adding OSD.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>