virtio-pci: S3 support

There's no difference in supporting S3 and S4 for virtio devices: the
vqs have to be re-created as the device has to be assumed to be reset at
restore-time.  Since S4 already handles this situation, we can directly
use the same code and callbacks for S3 support.

Signed-off-by: Amit Shah <amit.shah@redhat.com>