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 d0d7708ba29cbcc343364a46bff981e0ff88366f
  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: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Hervé Poussineau <hpoussin@reactos.org>
Message-id: 1453377386-10190-1-git-send-email-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
