hw/display: add ramfb, a simple boot framebuffer living in guest ram

hw/display: add ramfb, a simple boot framebuffer living in guest ram

The boot framebuffer is expected to be configured by the firmware, so it
uses fw_cfg as interface.  Initialization goes as follows:

  (1) Check whenever etc/ramfb is present.
  (2) Allocate framebuffer from RAM.
  (3) Fill struct RAMFBCfg, write it to etc/ramfb.

Done.  You can write stuff to the framebuffer now, and it should appear
automagically on the screen.

Note that this isn't very efficient because it does a full display
update on each refresh.  No dirty tracking.  Dirty tracking would have
to be active for the whole ram slot, so that wouldn't be very efficient
either.  For a boot display which is active for a short time only this
isn't a big deal.  As permanent guest display something better should be
used (if possible).

This is the ramfb core code.  Some windup is needed for display devices
which want have a ramfb boot display.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 20180613122948.18149-2-kraxel@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>