tests/qtest/virtio-ccw: Fix device presence checking

tests/qtest/virtio-ccw: Fix device presence checking

An apparent copy-paste error tests for the presence of the
virtio-rng-ccw device in order to perform tests on the virtio-scsi-ccw
device.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Message-ID: <20240106130121.1244993-1-sam@rfc1149.net>
Fixes: 65331bf5d1 ("tests/qtest: Check for virtio-ccw devices before  using them")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit c98873ee4a0c2694aac976ab9affcf55da8b7e61)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>