console: add qemu display registry, add gtk

Add a registry for user interfaces.  Add qemu_display_init and
qemu_display_early_init helper functions for display initialization.

Hook up gtk ui as first user.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20180301100547.18962-2-kraxel@redhat.com