10570 Need workaround to EFI boot on AMI BIOS

illumos/illumos-gate@fd7977362aae2eaa5dcb89671159f4fd82f22ca1
https://github.com/illumos/illumos-gate/commit/fd7977362aae2eaa5dcb89671159f4fd82f22ca1

https://www.illumos.org/issues/10570

Some BIOS types are very pick about the GPT header size.

Author: John Levon <john.levon@joyent.com>