virtio_pci: unregister virtio device at device remove

Make sure to call unregister_virtio_device() when a virtio device is removed.
Otherwise, virtio_pci.ko cannot be rmmod'd.

This was spotted by Marcelo Tosatti.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>