doc: highlight that "raw" is the only useful RBD format for QEMU

Small doc patch making clear that people should really be only using "raw" as their QEMU rbd format. This was previously briefly alluded to in the RBD/OpenStack doc, but not really explained.

Brief IRC discussion with @jdurgin on the subject:

(11:46:34 PM) fghaas: joshd, gregsfortytwo: could it be that the recommendation against using the qcow2 image format on rbd, and the fact that only raw images are supported for Qemu, aren't documented anywhere? Or is that no longer true and my knowledge is outdated?
(11:48:53 PM) joshd: fghaas: technically you can use qcow2, but it just adds overhead and makes live-migration with caching unsafe
(11:49:18 PM) joshd: fghaas: the docs could be more explicit about it though - it's just mentioned at the end of the qemu and openstack instructions iirc
(11:49:49 PM) fghaas: joshd, yes, plus the qemu examples all explicitly specify format=raw, but they don't really explain why
(11:51:46 PM) fghaas: also, if by "at the end of the qemu instructions" you refer to http://ceph.com/docs/next/rbd/qemu-rbd/, then that actually doesn't mention anything specific
(11:53:40 PM) joshd: yeah, I was thinking of http://ceph.com/docs/next/rbd/qemu-rbd/#running-qemu-with-rbd and http://ceph.com/docs/master/rbd/rbd-openstack/#booting-from-a-block-device
(02/11/2014 12:00:45 AM) fghaas: joshd: would you like a doc patch? :)
(12:01:05 AM) joshd: fghaas: of course!
(12:25:39 AM) fghaas: joshd: re the doc patch -- "qemu-img create -f rbd rbd:{pool-name}/{image-name} {size}" ... is that intentional? shouldn't that be "-f raw", because the driver is only concerned about the rbd: path prefix?
(12:26:52 AM) joshd: fghaas: yes, that should just be raw for clarity
(12:27:02 AM) fghaas: ok