virtio_console: silence a static checker warning

My static checker complains that this sprintf() can overflow but really
it can't.  Just silence the warning by using snprintf().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>