ui/console: move graphic fields to QemuGraphicConsole

Move fields specific to graphic console to the console subclass.

qemu_console_get_head() is adapated to accomodate QemuTextConsole, and
always returns 0.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-30-marcandre.lureau@redhat.com>