vl: Fix bogus error message for implied mon ID clashing

monitor_parse() desugars --monitor, --qmp and -qmp-pretty to --mon.
The ID it picks can clash with a user-specified ID.  When it happens,
the error message is misleading.

Reproducer:

    $ qemu --mon id=compat_monitor0 --monitor stdio

Message before the patch:

    duplicate chardev: compat_monitor0

There's no "duplicate chardev" here.  The problem is a duplicate
monitor ID.  Moreover, the message provides no clue which option
caused the problem.  The patch changes the message to:

    qemu: --monitor stdio: Duplicate ID 'compat_monitor0' for mon

monitor_parse() is also used for creating a default monitor, but
that's not done when the user specifies a monitor, so an ID clash is
impossible then.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

vl.c

@@ -2195,6 +2195,7 @@ static int mon_init_func(QemuOpts *opts, void *opaque)
 static void monitor_parse(const char *optarg, const char *mode, bool pretty)
 {
     static int monitor_device_index = 0;
+    Error *local_err = NULL;
     QemuOpts *opts;
     const char *p;
     char label[32];
@@ -2215,9 +2216,10 @@ static void monitor_parse(const char *optarg, const char *mode, bool pretty)
         }
     }
 
-    opts = qemu_opts_create(qemu_find_opts("mon"), label, 1, NULL);
+    opts = qemu_opts_create(qemu_find_opts("mon"), label, 1, &local_err);
     if (!opts) {
-        fprintf(stderr, "duplicate chardev: %s\n", label);
+        error_report("%s", error_get_pretty(local_err));
+        error_free(local_err);
         exit(1);
     }
     qemu_opt_set(opts, "mode", mode);