qemu-img: Convert invocation documentation to rST

qemu-img: Convert invocation documentation to rST

The qemu-img documentation is currently in qemu-nbd.texi in Texinfo
format, which we present to the user as:
 * a qemu-img manpage
 * a section of the main qemu-doc HTML documentation

Convert the documentation to rST format, and present it to the user as:
 * a qemu-img manpage
 * part of the interop/ Sphinx manual

The qemu-img rST document uses the new hxtool extension
to handle pulling rST fragments out of qemu-img-cmds.hx.

The documentation of the various options and commands is rather
muddled, with some options being described inside the relevant
command description and some in a more general section near the start
of the manual.  All the command synopses are replicated in the .hx
file and then again in the manual.  A lot of text is also duplicated
in the qemu-img.c code for the help text.  I have not attempted to
deal with any of this, but have simply transposed the existing
structure into rST.

As usual, there are some minor formatting changes but no
textual changes, except that as with one or two other conversions
I have dropped the 'see also' section since it's not very
informative and looks odd in the HTML.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200124162606.8787-6-peter.maydell@linaro.org