i386: Change X86CPUDefinition::model_id to const char*

It is valid to have a 48-character model ID on CPUID, however the
definition of X86CPUDefinition::model_id is char[48], which can
make the compiler drop the null terminator from the string.

If a CPU model happens to have 48 bytes on model_id, "-cpu help"
will print garbage and the object_property_set_str() call at
x86_cpu_load_def() will read data outside the model_id array.

We could increase the array size to 49, but this would mean the
compiler would not issue a warning if a 49-char string is used by
mistake for model_id.

To make things simpler, simply change model_id to be const char*,
and validate the string length using an assert() on
x86_register_cpudef_type().

Backports commit 4b220d88ba76fb2623ce4b8ba1f1eea66b82144e from qemu

qemu/target/i386/cpu.c

@@ -883,7 +883,7 @@ struct X86CPUDefinition {
     int model;
     int stepping;
     FeatureWordArray features;
-    char model_id[48];
+    const char *model_id;
     bool cache_info_passthrough;
 };
 
@@ -1094,6 +1094,7 @@ static X86CPUDefinition builtin_x86_defs[] = {
         // FEAT_1_EDX
             I486_FEATURES,
         },
+        "",
     },
     {
         "pentium",
@@ -1104,6 +1105,7 @@ static X86CPUDefinition builtin_x86_defs[] = {
         // FEAT_1_EDX
             PENTIUM_FEATURES,
         },
+        "",
     },
     {
         "pentium2",
@@ -1114,6 +1116,7 @@ static X86CPUDefinition builtin_x86_defs[] = {
         // FEAT_1_EDX
             PENTIUM2_FEATURES,
         },
+        "",
     },
     {
         "pentium3",
@@ -1124,6 +1127,7 @@ static X86CPUDefinition builtin_x86_defs[] = {
         // FEAT_1_EDX
             PENTIUM3_FEATURES,
         },
+        "",
     },
     {
         "athlon",
@@ -2526,6 +2530,9 @@ static void x86_register_cpudef_type(struct uc_struct *uc, X86CPUDefinition *def
         x86_cpu_cpudef_class_init,
     };
 
+    /* catch mistakes instead of silently truncating model_id when too long */
+    assert(def->model_id && strlen(def->model_id) <= 48);
+
     type_register(uc, &ti);
     g_free(typename);
 }