gtk: use qemu_chr_alloc() to allocate CharDriverState

The gd_vc_handler() callback is using g_malloc0() to
allocate the CharDriverState struct. As a result the
logfd field is getting initialized to 0, instead of
-1 when no logfile is requested.

The result is that when running

 $ qemu-system-i386 -nodefaults -chardev vc,id=mon0 -mon chardev=mon0

qemu duplicates all monitor output to stdout as well
as the GTK window.

Not using qemu_chr_alloc() was already a bug, but harmless
until this commit

  commit d0d7708b
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Mon Jan 11 12:44:41 2016 +0000

    qemu-char: add logfile facility to all chardev backends

which exposed the problem as a behaviour regression
Reported-by: NHervé Poussineau <hpoussin@reactos.org>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Tested-by: NHervé Poussineau <hpoussin@reactos.org>
Message-id: 1453377386-10190-1-git-send-email-berrange@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/gtk.c

@@ -1598,11 +1598,16 @@ static void gd_vc_chr_set_echo(CharDriverState *chr, bool echo)
 static int nb_vcs;
 static CharDriverState *vcs[MAX_VCS];
 
-static CharDriverState *gd_vc_handler(ChardevVC *unused, Error **errp)
+static CharDriverState *gd_vc_handler(ChardevVC *vc, Error **errp)
 {
+    ChardevCommon *common = qapi_ChardevVC_base(vc);
     CharDriverState *chr;
 
-    chr = g_malloc0(sizeof(*chr));
+    chr = qemu_chr_alloc(common, errp);
+    if (!chr) {
+        return NULL;
+    }
+
     chr->chr_write = gd_vc_chr_write;
     chr->chr_set_echo = gd_vc_chr_set_echo;