configure: Only check for audio drivers if system-mode is selected

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20210122204441.2145197-2-philmd@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>