vdpa: Allow VDPA to work on big-endian machine

After commit 0caed25cd1 vhost_vdpa_net_load_vlan() started seeing
VIRTIO_NET_F_CTRL_VLAN flag and making 4096 calls to the kernel with
VIRTIO_NET_CTRL_VLAN_ADD command. However, it forgot to convert the
16-bit VLAN IDs to LE format. On BE machine, the kernel calls failed
when they saw "VLAN IDs" greater than 4095, and QEMU then said:
"unable to start vhost net: 5: falling back on userspace virtio", and
VDPA became disabled.

Convert the VLAN ID to LE before putting it into virtio queue.

Fixes: 8f7e996748 ("vdpa: Restore vlan filtering state")
Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Message-ID: <20260427144746.1498-1-kshk@linux.ibm.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

net/vhost-vdpa.c

@@ -1200,9 +1200,10 @@ static int vhost_vdpa_net_load_single_vlan(VhostVDPAState *s,
                                            struct iovec *in_cursor,
                                            uint16_t vid)
 {
+    uint16_t vid_le = cpu_to_le16(vid);
     const struct iovec data = {
-        .iov_base = &vid,
-        .iov_len = sizeof(vid),
+        .iov_base = &vid_le,
+        .iov_len = sizeof(vid_le),
     };
     ssize_t r = vhost_vdpa_net_load_cmd(s, out_cursor, in_cursor,
                                         VIRTIO_NET_CTRL_VLAN,