virtio-scsi: fix hotplug ->reset() vs event race

virtio-scsi: fix hotplug ->reset() vs event race

There is a race condition during hotplug when iothread is used.  It
occurs because virtio-scsi may be processing command queues in the
iothread while the monitor performs SCSI device hotplug.

When a SCSI device is hotplugged the HotplugHandler->plug() callback is
invoked and virtio-scsi emits a rescan event to the guest.

If the guest submits a SCSI command at this point then it may be
cancelled before hotplug completes.  This happens because ->reset() is
called by hw/core/qdev.c:device_set_realized() after
HotplugHandler->plug() has been called and
hw/scsi/scsi-disk.c:scsi_disk_reset() purges all requests.

This patch uses the new HotplugHandler->post_plug() callback to emit the
rescan event after ->reset().  This eliminates the race conditions where
requests could be cancelled.

Reported-by: l00284672 <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Fam Zheng <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20180716083732[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>