qemu: add qemu ramfb support

qemu: add qemu ramfb support

Add support for qemu ramfb.  This is a simple boot framebuffer device,
with normal ram being used to back the framebuffer and fw_cfg being used
to configure the device.

Use case (on x86): boot display for vgpu devices (which neither emulate
vga nor have a vgabios).

Sharing fw_cfg code with seabios turned out to be difficuilt due to
various dependencies the code has on infrastructure which only seabios
has.  So include a copy of the code here, with those dependencies
removed and also stripped down because we don't need a non-dma fallback
here.

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