Merge remote-tracking branch 'remotes/pmaydell/tags/pull-docs-20200203' into staging

Merge remote-tracking branch 'remotes/pmaydell/tags/pull-docs-20200203' into staging

docs:
 * Fix Makefile concurrency bug where we could run Sphinx twice
   in parallel on the same manual (which makes it crash)
 * Support handling hxtool doc fragments for rST manuals
 * Convert qemu-img docs to rST
 * Convert qemu-trace-stap docs to rST
 * Convert virtfs-proxy-helper docs to rST

# gpg: Signature made Mon 03 Feb 2020 11:11:44 GMT
# gpg:                using RSA key E1A5C593CD419DE28E8315CF3C2525ED14360CDE
# gpg:                issuer "peter.maydell@linaro.org"
# gpg: Good signature from "Peter Maydell <peter.maydell@linaro.org>" [ultimate]
# gpg:                 aka "Peter Maydell <pmaydell@gmail.com>" [ultimate]
# gpg:                 aka "Peter Maydell <pmaydell@chiark.greenend.org.uk>" [ultimate]
# Primary key fingerprint: E1A5 C593 CD41 9DE2 8E83  15CF 3C25 25ED 1436 0CDE

* remotes/pmaydell/tags/pull-docs-20200203:
  virtfs-proxy-helper: Convert documentation to rST
  scripts/qemu-trace-stap: Convert documentation to rST
  qemu-img-cmds.hx: Remove texinfo document fragments
  qemu-img: Convert invocation documentation to rST
  qemu-img-cmds.hx: Add rST documentation fragments
  docs/sphinx: Add new hxtool Sphinx extension
  hxtool: Support SRST/ERST directives
  Makefile: Ensure we don't run Sphinx in parallel for manpages

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>