vhost-user: allow "vhost_set_vring" to wait for a reply

The "vhost_set_vring" function already centralizes the common parts of
"vhost_user_set_vring_num", "vhost_user_set_vring_base" and
"vhost_user_set_vring_enable". We'll want to allow some of those callers
to wait for a reply.

Therefore, rebase "vhost_set_vring" from just "vhost_user_write" to
"vhost_user_write_sync", exposing the "wait_for_reply" parameter.

This is purely refactoring -- there is no observable change. That's
because:

- all three callers pass in "false" for "wait_for_reply", which disables
  all logic in "vhost_user_write_sync" except the call to
  "vhost_user_write";

- the fds=NULL and fd_num=0 arguments of the original "vhost_user_write"
  call inside "vhost_set_vring" are hard-coded within
  "vhost_user_write_sync".

Cc: "Michael S. Tsirkin" <mst@redhat.com> (supporter:vhost)
Cc: Eugenio Perez Martin <eperezma@redhat.com>
Cc: German Maglione <gmaglione@redhat.com>
Cc: Liu Jiang <gerry@linux.alibaba.com>
Cc: Sergio Lopez Pascual <slp@redhat.com>
Cc: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Albert Esteve <aesteve@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20231002203221.17241-7-lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user.c

@@ -1160,7 +1160,8 @@ static int vhost_user_write_sync(struct vhost_dev *dev, VhostUserMsg *msg,
 
 static int vhost_set_vring(struct vhost_dev *dev,
                            unsigned long int request,
-                           struct vhost_vring_state *ring)
+                           struct vhost_vring_state *ring,
+                           bool wait_for_reply)
 {
     VhostUserMsg msg = {
         .hdr.request = request,
@@ -1169,13 +1170,13 @@ static int vhost_set_vring(struct vhost_dev *dev,
         .hdr.size = sizeof(msg.payload.state),
     };
 
-    return vhost_user_write(dev, &msg, NULL, 0);
+    return vhost_user_write_sync(dev, &msg, wait_for_reply);
 }
 
 static int vhost_user_set_vring_num(struct vhost_dev *dev,
                                     struct vhost_vring_state *ring)
 {
-    return vhost_set_vring(dev, VHOST_USER_SET_VRING_NUM, ring);
+    return vhost_set_vring(dev, VHOST_USER_SET_VRING_NUM, ring, false);
 }
 
 static void vhost_user_host_notifier_free(VhostUserHostNotifier *n)
@@ -1206,7 +1207,7 @@ static void vhost_user_host_notifier_remove(VhostUserHostNotifier *n,
 static int vhost_user_set_vring_base(struct vhost_dev *dev,
                                      struct vhost_vring_state *ring)
 {
-    return vhost_set_vring(dev, VHOST_USER_SET_VRING_BASE, ring);
+    return vhost_set_vring(dev, VHOST_USER_SET_VRING_BASE, ring, false);
 }
 
 static int vhost_user_set_vring_enable(struct vhost_dev *dev, int enable)
@@ -1224,7 +1225,7 @@ static int vhost_user_set_vring_enable(struct vhost_dev *dev, int enable)
             .num   = enable,
         };
 
-        ret = vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state);
+        ret = vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state, false);
         if (ret < 0) {
             /*
              * Restoring the previous state is likely infeasible, as well as