DisplayChangeListener gets a new QemuConsole field, which can be set to
non-NULL before registering.  This will pin the QemuConsole, so that
particular DisplayChangeListener will not follow console switches.

spice+gtk (which don't support text console input anyway) are switched
over to be pinned to console 0, which usually is the graphical display.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>