Provide a more direct interface to tell ZFS what the preferred handle

Provide a more direct interface to tell ZFS what the preferred handle
is. We tell the ZFS code now, and it checks rather than having a
callback to do the checks.

This will allow us to have a more graceful fallback code. In the
future, it's anticipated that we may fallback to a more global search
(or implement a command to do so) when reqeusted by the user, or we
detect a violation of the UEFI Boot Manager protocol severe enough to
warrant this backstop. For now, it just allows us to get rid of img as
a global.

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