efirt: add hooks for diverging EFI implementations

Introduce a set of hooks for MI EFI public functions, so that a new
implementation can be done. This will be used to implement the Xen PV
EFI interface that's used when running FreeBSD as a Xen dom0 from UEFI
firmware. Also make the efi_status_to_errno non-static since it will
be used to evaluate status return values from the PV interface.

No functional change indented.

Sponsored by: Citrix Systems R&D
Reviewed by: kib, imp
Differential revision: https://reviews.freebsd.org/D28620