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
diff --git a/MAINTAINERS b/MAINTAINERS
index db3cbc1..1f0bc72 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1574,6 +1574,7 @@
 F: hw/9pfs/
 X: hw/9pfs/xen-9p*
 F: fsdev/
+F: docs/interop/virtfs-proxy-helper.rst
 F: tests/qtest/virtio-9p-test.c
 T: git https://github.com/gkurz/qemu.git 9p-next