systemd: avoid string comparisons on dbus error messages

Add a virDBusErrorIsUnknownMethod helper so that callers
don't need todo string comparisons themselves to detect
standard error names.

src/libvirt_private.syms

@@ -1244,6 +1244,7 @@ virDBusCreateMethod;
 virDBusCreateMethodV;
 virDBusCreateReply;
 virDBusCreateReplyV;
+virDBusErrorIsUnknownMethod;
 virDBusGetSessionBus;
 virDBusGetSystemBus;
 virDBusHasSystemBus;

src/util/virdbus.c

@@ -1894,3 +1894,12 @@ void virDBusMessageUnref(DBusMessage *msg ATTRIBUTE_UNUSED)
     /* nothing */
 }
 #endif /* ! WITH_DBUS */
+
+bool virDBusErrorIsUnknownMethod(virErrorPtr err)
+{
+    return err->domain == VIR_FROM_DBUS &&
+        err->code == VIR_ERR_DBUS_SERVICE &&
+        err->level == VIR_ERR_ERROR &&
+        STREQ_NULLABLE("org.freedesktop.DBus.Error.UnknownMethod",
+                       err->str1);
+}

src/util/virdbus.h

@@ -74,4 +74,6 @@ void virDBusMessageUnref(DBusMessage *msg);
 
 int virDBusIsServiceEnabled(const char *name);
 int virDBusIsServiceRegistered(const char *name);
+
+bool virDBusErrorIsUnknownMethod(virErrorPtr err);
 #endif /* __VIR_DBUS_H__ */

src/util/virsystemd.c

@@ -281,8 +281,7 @@ int virSystemdCreateMachine(const char *name,
             goto cleanup;
 
         if (error.level == VIR_ERR_ERROR) {
-            if (STREQ_NULLABLE("org.freedesktop.DBus.Error.UnknownMethod",
-                               error.str1)) {
+            if (virDBusErrorIsUnknownMethod(&error)) {
                 VIR_INFO("CreateMachineWithNetwork isn't supported, switching "
                          "to legacy CreateMachine method for systemd-machined");
                 virResetError(&error);