cpu: Turn cpu_unassigned_access() into a CPUState hook

Use it for all targets, but be careful not to pass invalid CPUState.
cpu_single_env can be NULL, e.g. on Xen.

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

target-mips/cpu.c

@@ -80,6 +80,7 @@ static void mips_cpu_class_init(ObjectClass *c, void *data)
 
     cc->do_interrupt = mips_cpu_do_interrupt;
     cc->dump_state = mips_cpu_dump_state;
+    cpu_class_set_do_unassigned_access(cc, mips_cpu_unassigned_access);
 }
 
 static const TypeInfo mips_cpu_type_info = {