tap: forbid creating multiqueue tap when hub is used

Obviously, hub does not support multiqueue tap. So this patch forbids creating
multiple queue tap when hub is used to prevent the crash when command line such
as "-net tap,queues=2" is used.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

net/tap.c

@@ -693,6 +693,13 @@ int net_init_tap(const NetClientOptions *opts, const char *name,
     queues = tap->has_queues ? tap->queues : 1;
     vhostfdname = tap->has_vhostfd ? tap->vhostfd : NULL;
 
+    /* QEMU vlans does not support multiqueue tap, in this case peer is set.
+     * For -netdev, peer is always NULL. */
+    if (peer && (tap->has_queues || tap->has_fds || tap->has_vhostfds)) {
+        error_report("Multiqueue tap cannnot be used with QEMU vlans");
+        return -1;
+    }
+
     if (tap->has_fd) {
         if (tap->has_ifname || tap->has_script || tap->has_downscript ||
             tap->has_vnet_hdr || tap->has_helper || tap->has_queues ||