console: nicer initial screen

Now that we have a function to create a fancy DisplaySurface with a
message for the user, to handle non-existing graphics hardware, we
can make it more generic and use it for other things too.

This patch adds a text line to the in initial DisplaySurface, notifying
the user that the display isn't initialized yet by the guest.

You can see this in action when starting qemu with '-S'.  Also when
booting ovmf in qemu (which needs a few moments to initialize itself
before it initializes the vga).

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