This update brings dataplane to virtio-scsi (NOT yet 100% thread-safe, though, which makes it really, really experimental. It also brings asynchronous cancellation to the SCSI subsystem and implements it in virtio-scsi. This is a pretty important feature. Almost all the work here was done by Fam Zheng. I also included the virtio refcount fixes from Gonglei, because they had a small conflict with virtio-scsi dataplane. This pull request is using the new subkey 4E6B09D7.