Fix corner case in chardev udp: parameter

The missing '@' broke 'udp::<port>@:<port>' parsing.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit 39324ca4)

qemu-char.c

@@ -2317,7 +2317,7 @@ QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename)
         qemu_opt_set(opts, "backend", "udp");
         if (sscanf(p, "%64[^:]:%32[^@,]%n", host, port, &pos) < 2) {
             host[0] = 0;
-            if (sscanf(p, ":%32[^,]%n", port, &pos) < 1) {
+            if (sscanf(p, ":%32[^@,]%n", port, &pos) < 1) {
                 goto fail;
             }
         }