vhost-user: verify that number of queues is less than MAX_QUEUE_NUM

Fix QEMU crash when -netdev vhost-user,queues=n is passed with number
of queues greater than MAX_QUEUE_NUM.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
