Merge remote-tracking branch 'remotes/pmaydell/tags/pull-docs-20200225' into staging

 * create a new 'tools' manual, and move the documentation for
   qemu-img, qemu-nbd, virtfs-proxy-helper, qemu-trace-stap and
   virtiofsd to it

# gpg: Signature made Tue 25 Feb 2020 11:00:16 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-20200225:
  docs: Move tools documentation to tools manual
  docs: Move qemu-option-trace.rst.inc to docs/
  docs: Create new 'tools' manual

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