Currently, vdev_online() will only post sysevent if previous state was
"offline". It should also post the event when the state changes from "removed"
or "faulted" to "healthy" or "degraded".
This will fix the following scenario:
- pull disk from slot A
- check that hotspare has taken its place (if available)
- insert disk into slot B
- check that hotspare moved back to "avail" state (if spare was used)
The problem here is that we don't get any ESC_ZFS_VDEV_* notification and fail
to update the vdev FRU.