target/s390x/cpu_models: Fix missing ERRP_GUARD() for error_prepend()

As the comment in qapi/error, passing @errp to error_prepend() requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
...
* - It should not be passed to error_prepend(), error_vprepend() or
*   error_append_hint(), because that doesn't work with &error_fatal.
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.

ERRP_GUARD() could avoid the case when @errp is &error_fatal, the user
can't see this additional information, because exit() happens in
error_setg earlier than information is added [1].

In target/s390x/cpu_models.c, there are 2 functions passing @errp to
error_prepend() without ERRP_GUARD():
- check_compatibility()
- s390_realize_cpu_model()

Though both their @errp parameters point to their callers' local @err
virables and don't cause the issue as [1] said, to follow the
requirement of @errp, also add missing ERRP_GUARD() at their beginning.

[1]: Issue description in the commit message of commit ae7c80a7bd
     ("error: New macro ERRP_GUARD()").

Cc: David Hildenbrand <david@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: qemu-s390x@nongnu.org
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-ID: <20240311033822.3142585-30-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

target/s390x/cpu_models.c

@@ -503,6 +503,7 @@ static void error_prepend_missing_feat(const char *name, void *opaque)
 static void check_compatibility(const S390CPUModel *max_model,
                                 const S390CPUModel *model, Error **errp)
 {
+    ERRP_GUARD();
     S390FeatBitmap missing;
 
     if (model->def->gen > max_model->def->gen) {
@@ -566,6 +567,7 @@ S390CPUModel *get_max_cpu_model(Error **errp)
 
 void s390_realize_cpu_model(CPUState *cs, Error **errp)
 {
+    ERRP_GUARD();
     Error *err = NULL;
     S390CPUClass *xcc = S390_CPU_GET_CLASS(cs);
     S390CPU *cpu = S390_CPU(cs);