hw/display/ramfb: initialize fw-config space with xres/ yres

If xres / yres were specified in QEMU command line, write them as an initial
resolution to the fw-config space on guest reset, which a later BIOS / OVMF
patch can take advantage of.

Signed-off-by: HOU Qiming <hqm03ster@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-id: 20190513115731.17588-4-marcel.apfelbaum@gmail.com
[fixed malformed patch]
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>