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

Explain why people should be using the "raw" image format for RBD
volumes created for use by QEMU: using any other format adds only
overhead, but no extra value (since RBDs are also CoW and
thin-provisioned), plus the Qcow2 storage driver is not migration safe
when caching is enabled, whereas the RBD driver is.

Also, fix a minor glitch in the example qemu-img commands ("-f rbd"
and "-O rbd" should really be "-f raw" and "-O raw").

Finally, drop the "-f" option altogether on qemu-img commands where it
makes no sense (info and resize).

Signed-off-by: Florian Haas <florian@hastexo.com>
Merge pull request #1209 from fghaas/master

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