shix: Catch CPU initialization errors

Print an error message as done for the r2d machine and exit.

Signed-off-by: Andreas Färber <afaerber@suse.de>
sh7750: Change cpu field type to SuperHCPU

This brings us a step closer to QOM'ified SH7750 SoC and
fixes b350ab75 assuming SuperHCPU type for SUPERH_CPU_GET_CLASS().

Signed-off-by: Andreas Färber <afaerber@suse.de>