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 659f195)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/9pfs/9p-proxy.c

@@ -1139,10 +1139,10 @@ static int proxy_parse_opts(QemuOpts *opts, FsDriverEntry *fs, Error **errp)
     }
     if (socket) {
         fs->path = g_strdup(socket);
-        fs->export_flags = V9FS_PROXY_SOCK_NAME;
+        fs->export_flags |= V9FS_PROXY_SOCK_NAME;
     } else {
         fs->path = g_strdup(sock_fd);
-        fs->export_flags = V9FS_PROXY_SOCK_FD;
+        fs->export_flags |= V9FS_PROXY_SOCK_FD;
     }
     return 0;
 }