incomplete handling of virtio scsi events

[wjhun]

The virtio_scsi driver enqueues buffers to the eventq but does not actually handle them on completion. The following events could be reported to the driver which require some action to be taken:

• transport reset: This may occur as a result of hotplug events or a device being reset. Sense codes cannot be relied on when new devices or busses appear, so this event needs to be handled.
• asynchronous notification: The driver doesn't currently issue async notification queries, so this may not need special handling right now.
• LUN parameter change: Device capacity or cache mode has changed. A change in capacity would need some special handling by the driver.

Additionally, virtio_scsi_event_complete could simply reuse the closure rather than calling virtio_scsi_enqueue_event to create a new one.

See "5.6.6.3 Device Operation: eventq" in the virtio-1.1 spec for more information. It may be worth doing a little survey of the cloud providers we support and see what types of hotplug or parameter change events might take place.

[sanderssj]

Partially addressed by #1628