xen/efi: introduce a PV interface for EFI run time services for dom0

FreeBSD when running as a dom0 under Xen is not supposed to access the
run time services directly, and instead should proxy the calls through
Xen using an hypercall interface that exposes access to selected run
time services.

Implement the efirt interface on top of the Xen provided hypercalls.

Sponsored by: Citrix Systems R&D
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D28621