OpenZFS 7136 - ESC_VDEV_REMOVE_AUX ought to always include vdev information

OpenZFS 7136 - ESC_VDEV_REMOVE_AUX ought to always include vdev information

Authored by: Alan Somers <asomers@gmail.com>
7115 6922 generates ESC_ZFS_VDEV_REMOVE_AUX a bit too often
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7136
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/b72b6bb
Closes #5691

Porting notes:
- Functionally this patch behaves the same as the OpenZFS
  version but it was adapted because because ZoL doesn't
  have the same illumos sysevent_t infrastructure and functionality.