target-m68k: Detect attempt to instantiate non-CPU type in cpu_init()

Consolidate model checking into a new m68k_cpu_class_by_name().

If the name matches an existing type, also check whether that type is
(a sub-type of) TYPE_M68K_CPU.

This fixes, e.g., -cpu ide-hd asserting.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/target-m68k/helper.c b/target-m68k/helper.c
index 097fc78..f66e12b 100644
--- a/target-m68k/helper.c
+++ b/target-m68k/helper.c
@@ -97,12 +97,14 @@
 {
     M68kCPU *cpu;
     CPUM68KState *env;
+    ObjectClass *oc;
     static int inited;
 
-    if (object_class_by_name(cpu_model) == NULL) {
+    oc = cpu_class_by_name(TYPE_M68K_CPU, cpu_model);
+    if (oc == NULL) {
         return NULL;
     }
-    cpu = M68K_CPU(object_new(cpu_model));
+    cpu = M68K_CPU(object_new(object_class_get_name(oc)));
     env = &cpu->env;
 
     if (!inited) {