hw/9pfs: Read-only support for 9p export

A new fsdev parameter "readonly" is introduced to control accessing 9p export.
"readonly" can be used to specify the access type. By default "rw" access
is given to 9p export.

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
diff --git a/fsdev/qemu-fsdev.c b/fsdev/qemu-fsdev.c
index 5977bcc..27d10cb 100644
--- a/fsdev/qemu-fsdev.c
+++ b/fsdev/qemu-fsdev.c
@@ -35,7 +35,7 @@
     const char *path = qemu_opt_get(opts, "path");
     const char *sec_model = qemu_opt_get(opts, "security_model");
     const char *writeout = qemu_opt_get(opts, "writeout");
-
+    bool ro = qemu_opt_get_bool(opts, "readonly", 0);
 
     if (!fsdev_id) {
         fprintf(stderr, "fsdev: No id specified\n");
@@ -86,6 +86,11 @@
             fsle->fse.export_flags |= V9FS_IMMEDIATE_WRITEOUT;
         }
     }
+    if (ro) {
+        fsle->fse.export_flags |= V9FS_RDONLY;
+    } else {
+        fsle->fse.export_flags &= ~V9FS_RDONLY;
+    }
 
     if (strcmp(fsdriver, "local")) {
         goto done;