Merge remote-tracking branch 'remotes/gkurz/tags/9p-next-2020-03-10' into staging

Fix the proxy fsdev so that it honours "readonly" and "writeout".

# gpg: Signature made Tue 10 Mar 2020 15:16:39 GMT
# gpg:                using RSA key B4828BAF943140CEF2A3491071D4D5E5822F73D6
# gpg: Good signature from "Greg Kurz <groug@kaod.org>" [full]
# gpg:                 aka "Gregory Kurz <gregory.kurz@free.fr>" [full]
# gpg:                 aka "[jpeg image of size 3330]" [full]
# Primary key fingerprint: B482 8BAF 9431 40CE F2A3  4910 71D4 D5E5 822F 73D6

* remotes/gkurz/tags/9p-next-2020-03-10:
  9p/proxy: Fix export_flags

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