QEMU Windows COM port filenames not recognized i.e. \\.\COM19 or \\.\CNCA0
Steps to reproduce:
1. Run qemu-system-arm with the comand line above.
2. QEMU fails with `qemu-system-arm.exe: -gdb \\.\CNCA8: '\\.\CNCA8' is not a valid char driver`
3. ```qemu-system-arm.exe -machine mps2-an500 -gdb \\.\COM19
qemu-system-arm.exe: -gdb \\.\COM19: '\\.\COM19' is not a valid char driver
```
Additional information:
Windows allows COM ports numbered 10 and higher to be prefixed with a `\\.\` escape as in `\\.\COM17`. Such COM port assignments are not uncommon when a plurality of USB serial adapters.
Equally problematic are virtual COM port designations such as `\\.\CNCA8` created by the Windows 10x64 driver package known as `com0com`: https://pete.akeo.ie/2011/07/com0com-signed-drivers.html

Upon checking the source pulled from the Github mirror an initial fix was to simply modify /chardev/char.c, but this appears insufficient. Sadly.

Please ask if more information is required. I am actively working on extending an existing QEMU machine emulation. A patch to fix this problem is below. Please comment if applicable.

Jerry.

```
diff --git a/chardev/char.c b/chardev/char.c
index 3c43fb1278..7a3f342c72 100644
--- a/chardev/char.c
+++ b/chardev/char.c
@@ -418,6 +418,13 @@ QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename,
         qemu_opt_set(opts, "path", filename, &error_abort);
         return opts;
     }
+       // JME
+    if (strstart(filename, "\\\\.\\", NULL)) {
+        qemu_opt_set(opts, "backend", "serial", &error_abort);
+        qemu_opt_set(opts, "path", filename, &error_abort);
+        return opts;
+    }
+
     if (strstart(filename, "file:", &p)) {
         qemu_opt_set(opts, "backend", "file", &error_abort);
         qemu_opt_set(opts, "path", p, &error_abort);

```
/label ~"kind::Bug"