9p/proxy: Fix export_flags

9p/proxy: Fix export_flags

The common fsdev options are set by qemu_fsdev_add() before it calls
the backend specific option parsing code. In the case of "proxy" this
means "writeout" or "readonly" were simply ignored. This has been
broken from the beginning.

Reported-by: Stéphane Graber <stgraber@ubuntu.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <158349633705.1237488.8895481990204796135.stgit@bahia.lan>
(cherry picked from commit 659f1953281bcfa5ac217e42877d7d3c32eeea38)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>