hw/display/ramfb: lock guest resolution after it's set

hw/display/ramfb: lock guest resolution after it's set

Only allow one resolution change per guest boot, which prevents a
crash when the guest writes garbage to the configuration space (e.g.
when rebooting).

Signed-off-by: HOU Qiming <hqm03ster@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-id: 20190513115731.17588-3-marcel.apfelbaum@gmail.com
[fixed malformed patch]
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>