Windows: can we keep offset+length separate?

It would be nice to not have to encode the device name with
but rather, keep offset+length in the "rn" struct. But in
zpool_find_import_impl() the rn list is converted into nvlists, and
freed. We would need to also copy offset+length into nvlists.
(This current attempt tries to locate the partition meant in the
update_vdev_config_dev_strs() by re-reading the EFI label and matching
guid. This falls apart with "zpool create", as there is no partition or
matching guid yet.

So we can either add the nvlist properties, and attempt to pass it
along. Hopefully the nvlists are not stored on disks.

Or we comment out the last-ditch open() test done in
zpool_find_import_impl(), or fix up
open() to handle #offset#size#name.

This commit will be reverted next.