Fix zvol export LOR based on FreeBSD patch for similar LOR:
https://github.com/freebsd/freebsd/commit/1d8972b3f353f986eb5b85bc108b1c0d946d3218
hopefully to address the problem with pool-in-zvol-export-deadlock
in issue #157
ZVOL: Set our devices in IOKit to be type 'Virtual'
libefi: Fetch device type and check for virtual
Fix the logic fubar
Attempt to detect inactive spa for zvol export
Use spa_suspended() API
spa_open has namespace mutex recursion checks for vdev unloading time, but
on OSX, this comes as a separate thread (IOkit) so we need a little extra
protection.