Make virtio-pci building conditional again

Make virtio-pci building conditional again

Commit b305b9d7d6990e492966ffb1fdf619482adeb7e2 made building of virtio-pci
conditional and not enabled on S390x, because it collides with the S390 bus.

Commit 087431d1d1bf4e785edfa89e8cd05fcdac558dc3 accidentially reverted that
behavior, breaking S390x again.

So here's a follow-up patch disabling building of virtio-pci on S390x again.

This unbreaks the S390x target.

Signed-off-by: Alexander Graf <[email protected]>
Signed-off-by: Aurelien Jarno <[email protected]>