tests/acpi: allow override blkdev

microvm needs virtio-blk instead of ide.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id: 20200915120909.20838-19-kraxel@redhat.com