Emit ESC_ZFS_VDEV_REMOVE_AUX after removing an aux device
ZFS does not do a config_sync after removing an aux (spare, log, or cache) device. AFAICT this isn't being done because it is slow and was deemed unnecessary. However, it should be such a rare operation that speed doesn't matter, and not doing it results in two problems:
1) It is theoretically possible to remove an aux device from one pool and attach it to another, then lose power. When power is restored, both pools would think that they own the aux device.
2) Removal of the aux device doesn't send any useful sysevents to userland.