Add the rST versions of the documentation fragments.
Once we've converted qemu-img.texi to rST we can delete
the texi fragments; for the moment we leave them in place.

(Commit created with the aid of emacs query-replace-regexp
from "@var{\([^}]*\)}" to "\,(upcase \1))".)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200124162606.8787-5-peter.maydell@linaro.org