nbd: force read-only export for read-only devices

This is the desired behavior for HMP, but it is a better choice for QMP as well.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

blockdev-nbd.c

@@ -93,6 +93,13 @@ void qmp_nbd_server_add(const char *device, bool has_writable, bool writable,
         return;
     }
 
+    if (!has_writable) {
+        writable = true;
+    }
+    if (bdrv_is_read_only(bs)) {
+        writable = false;
+    }
+
     exp = nbd_export_new(bs, 0, -1, writable ? 0 : NBD_FLAG_READ_ONLY,
                          nbd_server_put_ref);