QEMU Windows COM port setup dialog always invoked and fails if none is available (USB or virtual serial port hardware)
Description of problem:
The Windows backend serial port in `chardev/char-win.c` always calls `CommConfigDialog()`. This should display a COM port configuration dialog which does (and can) not persist the COM port settings. If the COM port does not support this action (see https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-commconfigdialoga) then the function fails.
Steps to reproduce:
1. Currently not possible with QEMU releases as QEMU does not recognize extended COM port specifications like `\\.\COM19` or `\\.\CNCA0`
Additional information:
See https://support.microsoft.com/en-gb/topic/howto-specify-serial-ports-larger-than-com9-db9078a5-b7b6-bf00-240f-f749ebfd913e for details on COM port filenames.

I have a patch which 'fixes' this problem by setting the nominated COM port to defaults of `115200,8,N,0` which seems perfectly sensible in 2024. Please contact me for more details. A git diff shown below (with extensive error reporting)

N.B. Markodown will destroy formatting!

```
diff --git a/chardev/char-win.c b/chardev/char-win.c
index d4fb44c4dc..a05896ffe9 100644
--- a/chardev/char-win.c
+++ b/chardev/char-win.c
@@ -96,12 +96,24 @@ int win_chr_serial_init(Chardev *chr, const char *filename, Error **errp)
     s->file = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, 0, NULL,
                       OPEN_EXISTING, FILE_FLAG_OVERLAPPED, 0);
     if (s->file == INVALID_HANDLE_VALUE) {
+        {
+            char buffer[1024] = { 0 };
+            DWORD dw = GetLastError();
+            sprintf_s(buffer, 1024, "%s(%d) Error: %d 0x%x %s\r\n", __FILE__, __LINE__, dw, dw, filename);
+            OutputDebugString(buffer);
+        }
         error_setg_win32(errp, GetLastError(), "Failed CreateFile");
         s->file = NULL;
         goto fail;
     }

     if (!SetupComm(s->file, NRECVBUF, NSENDBUF)) {
+        {
+            char buffer[1024] = { 0 };
+            DWORD dw = GetLastError();
+            sprintf_s(buffer, 1024, "%s(%d) Error: %d 0x%x %s\r\n", __FILE__, __LINE__, dw, dw, filename);
+            OutputDebugString(buffer);
+        }
         error_setg(errp, "Failed SetupComm");
         goto fail;
     }
@@ -110,9 +122,31 @@ int win_chr_serial_init(Chardev *chr, const char *filename, Error **errp)
     size = sizeof(COMMCONFIG);
     GetDefaultCommConfig(filename, &comcfg, &size);
     comcfg.dcb.DCBlength = sizeof(DCB);
-    CommConfigDialog(filename, NULL, &comcfg);
-
+#if 1
+    // JME hardwire. There seems to be no mechanism to simply specify serial port options
+    comcfg.dcb.BaudRate = 115200;
+    comcfg.dcb.Parity = NOPARITY;
+    comcfg.dcb.StopBits = ONESTOPBIT;
+    comcfg.dcb.ByteSize = 8;
+#else
+    {
+        BOOL ret = CommConfigDialog(filename, NULL, &comcfg);
+        if (!ret)
+        {
+            char buffer[1024] = { 0 };
+            DWORD dw = GetLastError();
+            sprintf_s(buffer, 1024, "%s(%d) Error: %d 0x%x %s\r\n", __FILE__, __LINE__, dw, dw, filename);
+            OutputDebugString(buffer);
+        }
+    }
+#endif
     if (!SetCommState(s->file, &comcfg.dcb)) {
+        {
+            char buffer[1024]={0};
+            DWORD dw = GetLastError();
+            sprintf_s(buffer,1024,"%s(%d) Error: %d 0x%x %s\r\n",__FILE__,__LINE__,dw,dw, filename);
+            OutputDebugString(buffer);
+        }
         error_setg(errp, "Failed SetCommState");
         goto fail;
     }
```

/label ~"kind::Bug"