virtfs-proxy-helper: Convert documentation to rST

virtfs-proxy-helper: Convert documentation to rST

The virtfs-proxy-helper documentation is currently in
fsdev/qemu-trace-stap.texi in Texinfo format, which we
present to the user as:
 * a virtfs-proxy-helper manpage
 * but not (unusually for QEMU) part of the HTML docs

Convert the documentation to rST format that lives in
the docs/ subdirectory, and present it to the user as:
 * a virtfs-proxy-helper manpage
 * part of the interop/ Sphinx manual

There are minor formatting changes to suit Sphinx, but no
content changes. In particular I've split the -u and -g
options into each having their own description text.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Greg Kurz <groug@kaod.org>
Message-id: 20200124162606.8787-9-peter.maydell@linaro.org