host: When creating zpools, do not mount them.

The default behavior is the mount them as "/$poolname" in the root of the filesystem, and this is overly global and very constraining.

Signed-off-by: Eric Myhre <eric@flynn.io>
host: Until further notice, outright panic if zpool destruction fails; quietly failing to destroy a zpool leaves a stateful mess that is likely to bork a future operation.

Signed-off-by: Eric Myhre <eric@flynn.io>
host: When testing zfs, clean up all fd's correctly.  Unmounting a zpool will fail due to umount failing if there ere are open filedescriptions.

While this seems to imply something of a challenge for stability, fortunately, we're working on a containment system... so sequencing shutdown in order to make this a non-issue should be readily possible.

Signed-off-by: Eric Myhre <eric@flynn.io>
WIP

Signed-off-by: Eric Myhre <eric@flynn.io>