Added AIO_CHECK_EVENT to pdp11_cpu to properly digest the asynchronous event queue in a timely manner (with the desired interrupt latency) for asynch I/O
The Massbus byte count register was incorrectly presumed to be constant during a complete I/O transfer request (it can change due to DMA activity).  Prior to the conversion to use the sim_disk library, the Massbus byte count was only read once.  After the code restructuring it was read early in the processing and again later in transfer processing.  The subsequent read returned a different value which ultimately caused the problems.  This fixes issue #30.