doc: Added more robust usage example for uid and conf for QEMU.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
Merge pull request #2426 from ceph/wip-doc-qemu

doc: Added more robust usage example for uid and conf for QEMU.

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>