target-s390: Don't compile in virtio-pci

target-s390: Don't compile in virtio-pci

As soon as virtio-pci.c gets compiled and used on S390 the internal qdev magic
gets confused and tries to give us PCI devices instead of S390 virtio devices.

Since we don't have PCI on S390, we can safely not compile virtio-pci at all.

In order to do this I added a new config option "CONFIG_VIRTIO_PCI" that I
enabled for every platform except S390. Thanks to this the change should be a
complete nop for every other platform.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>