If a register writes back XXXXX, print an error message from the simulator during cosimulation.
Change how memory size is computed to be consistent.
Set axi_bvalid so we don't hang on writes.
Remove delays from sim_sdram.  Fix a bug where a fixed width (16 bits) was assumed for dq in sim_sdram when floating for input, resulting in an X.
Exit simulator when X is input to sim_sdram during write burst.