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>

include/hw/display/ramfb.h

@@ -0,0 +1,9 @@
+#ifndef RAMFB_H
+#define RAMFB_H
+
+/* ramfb.c */
+typedef struct RAMFBState RAMFBState;
+void ramfb_display_update(QemuConsole *con, RAMFBState *s);
+RAMFBState *ramfb_setup(Error **errp);
+
+#endif /* RAMFB_H */