qemu: monitor: json: Refactor error code class checker

Split out the function that checks the actual error class string into a
separate helper as it will be useful later and refactor
qemuMonitorJSONHasError to return bool type and remove few useless
checks.

Basically virJSONValueObjectHasKey are useless here since the next call
to virJSONValueObjectGet is checking the return value again (which can't
fail at that point). By removing the first check we save a function
call.

src/qemu/qemu_monitor_json.c

@@ -400,31 +400,27 @@ qemuMonitorJSONCheckError(virJSONValuePtr cmd,
 }
 
 
-static int
+static bool
+qemuMonitorJSONErrorIsClass(virJSONValuePtr error,
+                            const char *klass)
+{
+    return STREQ_NULLABLE(virJSONValueObjectGetString(error, "class"), klass);
+}
+
+
+static bool
 qemuMonitorJSONHasError(virJSONValuePtr reply,
                         const char *klass)
 {
     virJSONValuePtr error;
-    const char *thisklass;
-
-    if (!virJSONValueObjectHasKey(reply, "error"))
-        return 0;
 
-    error = virJSONValueObjectGet(reply, "error");
-    if (!error)
-        return 0;
-
-    if (!virJSONValueObjectHasKey(error, "class"))
-        return 0;
-
-    thisklass = virJSONValueObjectGetString(error, "class");
+    if (!(error = virJSONValueObjectGet(reply, "error")))
+        return false;
 
-    if (!thisklass)
-        return 0;
-
-    return STREQ(klass, thisklass);
+    return qemuMonitorJSONErrorIsClass(error, klass);
 }
 
+
 /* Top-level commands and nested transaction list elements share a
  * common structure for everything except the dictionary names.  */
 static virJSONValuePtr ATTRIBUTE_SENTINEL