console: Hook QemuConsoles into qom tree

Put them named "console[$index]" below "/backend", so you can
list & inspect them via QMP.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1372150171-8707-1-git-send-email-kraxel@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>