Retain error in newVZFileSerialPortAttachment

If VZFileSerialPortAttachment:initWithURL returns an error, we need to
call `retain` on it, otherwise it will be freed when exiting from the
@autoreleasepool block.

This fixes TestNonExistingFileSerialPortAttachment() which is
crashing before this commit:

```
% go test .
fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x2 addr=0xcf6cce7eb130 pc=0x19e33a5a0]

runtime stack:
runtime.throw({0x100ce0ded?, 0x100ccf234?})
	/usr/local/go/src/runtime/panic.go:1047 +0x40 fp=0x16f293700 sp=0x16f2936d0 pc=0x100ba2870
runtime.sigpanic()
	/usr/local/go/src/runtime/signal_unix.go:819 +0x1e4 fp=0x16f293730 sp=0x16f293700 pc=0x100bb9754

goroutine 19 [syscall]:
runtime.cgocall(0x100ccf214, 0x14000040d98)
	/usr/local/go/src/runtime/cgocall.go:158 +0x54 fp=0x14000040d60 sp=0x14000040d20 pc=0x100b6fbc4
github.com/Code-Hex/vz/v2._Cfunc_convertNSError2Flat(0x6000022cb120)
	_cgo_gotypes.go:344 +0x3c fp=0x14000040d90 sp=0x14000040d60 pc=0x100ccc11c
github.com/Code-Hex/vz/v2.newNSError.func1(0x1010c0a68?)
	/Users/teuf/dev/vz/objcutil.go:234 +0x48 fp=0x14000040df0 sp=0x14000040d90 pc=0x100cccd58
github.com/Code-Hex/vz/v2.newNSError(0x0?)
	/Users/teuf/dev/vz/objcutil.go:234 +0x30 fp=0x14000040e80 sp=0x14000040df0 pc=0x100cccbe0
github.com/Code-Hex/vz/v2.NewFileSerialPortAttachment({0x100cd9765?, 0x100be659c?}, 0x68?)
	/Users/teuf/dev/vz/console.go:92 +0x104 fp=0x14000040f10 sp=0x14000040e80 pc=0x100ccc6c4
github.com/Code-Hex/vz/v2.TestNonExistingFileSerialPortAttachment(0x0?)
	/Users/teuf/dev/vz/error_test.go:10 +0x30 fp=0x14000040f60 sp=0x14000040f10 pc=0x100ccbf10
testing.tRunner(0x14000107040, 0x100d81878)
	/usr/local/go/src/testing/testing.go:1446 +0x10c fp=0x14000040fb0 sp=0x14000040f60 pc=0x100c2d10c
testing.(*T).Run.func1()
	/usr/local/go/src/testing/testing.go:1493 +0x2c fp=0x14000040fd0 sp=0x14000040fb0 pc=0x100c2de4c
runtime.goexit()
	/usr/local/go/src/runtime/asm_arm64.s:1165 +0x4 fp=0x14000040fd0 sp=0x14000040fd0 pc=0x100bd4834
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1493 +0x300
[snip]
```

This fixes Code-Hex#56

virtualization.m

@@ -518,10 +518,14 @@ void setStreamsVZVirtioSoundDeviceConfiguration(void *audioDeviceConfiguration,
         @autoreleasepool {
             NSString *filePathNSString = [NSString stringWithUTF8String:filePath];
             NSURL *fileURL = [NSURL fileURLWithPath:filePathNSString];
+            NSError *_Nullable *_Nullable err = (NSError *_Nullable *_Nullable)error;
             ret = [[VZFileSerialPortAttachment alloc]
                 initWithURL:fileURL
                      append:(BOOL)shouldAppend
-                      error:(NSError *_Nullable *_Nullable)error];
+                      error:err];
+            if (err != nil && *err != nil) {
+                [*err retain];
+            }
         }
         return ret;
     }