ui/console: specialize console_lookup_unused()

graphics_console_init() is expected to return a graphic console.

The function doesn't need to be exported.

We are going to specialize further QemuGraphicConsole & QemuTextConsole.
The two will not be interchangeable anymore.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-24-marcandre.lureau@redhat.com>