rbd: assorted cli arg parsing fixes

rbd: assorted cli arg parsing fixes

- import-diff needs image-spec, yet OPT_IMPORT_DIFF was in the list of
  commands that don't need imgname

- check for lock id after imgname, so that if imgname isn't specified
  we fail with imgname rather than lock id error message

- error out if locker isn't specified

- move feature-name validity check after imgname checks so that if
  imgname is malformed we fail with imgname rather than feature error
  message

- error out if image-meta key or value isn't specified

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>