util/error: make func optional

The function name is not available in Rust, so make it optional.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qapi/error-internal.h

@@ -18,6 +18,8 @@ struct Error
 {
     char *msg;
     ErrorClass err_class;
+
+    /* Used for error_abort only, may be NULL. */
     const char *func;
 
     /*