virtio - Remove explicit load of virtio.ko. Remove device virtio_pci line.

virtio - Remove explicit load of virtio.ko. Remove device virtio_pci line.

* virtio.ko never needs to be loaded explicitly, since virtio_pci.ko
  depends on it.

* Also get rid of device virtio_pci line in the kernel config. Similar to
  other buses, link virtio_pci statically into the kernel when both
  device virtio and device pci are configured.