hw/virtio: Use CONFIG_VIRTIO_PCI switch instead of CONFIG_PCI

hw/virtio: Use CONFIG_VIRTIO_PCI switch instead of CONFIG_PCI

For downstream s390x builds, we'd like to be able to build QEMU with
CONFIG_VIRTIO_PCI disabled (since virtio-ccw is used here instead),
but still with CONFIG_PCI enabled. This currently fails since the
virtio-*-pci.o files are still included in the build, but virtio-pci.o
is missing. Use the right config switch CONFIG_VIRTIO_PCI to exclude
the virtio-*-pci.o files from the build.

Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>