config(qemu): fix serial hardware not saved

Configuration/UTMQemuConfiguration+Arguments.swift

@@ -185,7 +185,7 @@ import Virtualization // for getting network interfaces
                 f("chardev:term\(i)")
             case .manualDevice:
                 f("-device")
-                f("\(serials[i].hardware!.rawValue),chardev=term\(i)")
+                f("\(serials[i].hardware?.rawValue ?? "invalid"),chardev=term\(i)")
             case .monitor:
                 f("-mon")
                 f("chardev=term\(i),mode=readline")

Configuration/UTMQemuConfigurationSerial.swift

@@ -78,6 +78,7 @@ struct UTMQemuConfigurationSerial: Codable, Identifiable {
         var container = encoder.container(keyedBy: CodingKeys.self)
         try container.encode(mode, forKey: .mode)
         try container.encode(target, forKey: .target)
+        try container.encodeIfPresent(hardware?.asAnyQEMUConstant(), forKey: .hardware)
         // only save relevant settings
         switch mode {
         case .builtin: