Rather than checking if the machine is an s390x to use virtio-blk-ccw
instead of virtio-blk-pci, use the alias virtio-blk that is set to
the expected target.

This also enables the use of virtio-blk-device for targets without
PCI or CCW.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210319202335.2397060-4-laurent@vivier.eu>
Message-Id: <20210323165308.15244-19-alex.bennee@linaro.org>