vhost-user: verify that number of queues is non-zero

Fix QEMU crash when -netdev type=vhost-user,queues=n is passed
with zero number of queues.

Signed-off-by: Victor Kaplansky <victork@redhat.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>
