OpenZFS 5561 - support root pools on EFI/GPT partitioned disks

Reviewed by: Jean McCormack <jean.mccormack@nexenta.com>
Reviewed by: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/5561
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/1a902ef
Closes #5672