tests/acceptance/boot_linux_console: Do not use VGA on Clipper machine

As we only read the serial console, we don't need to force a
VGA display. This fixes when running a binary built with
--without-default-devices:

  ERROR: qemu-system-alpha: standard VGA not available

We also need the '-nodefaults' argument to avoid:

  ERROR: qemu-system-alpha: Unsupported NIC model: e1000

Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200129212345.20547-27-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>