chardev-frontends need to explictly check, increase and decrement the
avail_connections "property" of the chardev when they are not using a
qdev-chardev-property for the chardev.

This fixes things like:
qemu-kvm -chardev stdio,id=foo -device isa-serial,chardev=foo \
  -mon chardev=foo

Working, where they should fail. Most of the changes here are due to
old hardware emulation code which is using serial_hds directly rather then
a qdev-chardev-property.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Message-id: 1364412581-3672-3-git-send-email-hdegoede@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>