tests/functional: Use the 'none' machine for the VNC test

tests/functional: Use the 'none' machine for the VNC test

The VNC test currently fails if the default machine ("pc" for x86)
has not been compiled into the binary. Since we also can test VNC
when QEMU just shows the default monitor, let's avoid this problem
by simply using the "none" machine (which is always available)
here instead.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250603103449.32499-1-thuth@redhat.com>