HEADS UP: FreeBSD/Xen dom0 UEFI support

[Roger Pau Monné]

Hello,

Since commit 97527e9c4fd37140 on main branch FreeBSD should be able to
boot and work as a Xen dom0 from UEFI.

Booting from UEFI also requires the usage of xen-kernel 4.14.1_1,
previous versions of xen-kernel won't boot correctly under UEFI.

The way to setup the system is exactly the same as when using BIOS,
xen_kernel and xen_cmdline options should be set in loader.conf, see:

https://docs.freebsd.org/en_US.ISO8859-1/books/handbook/virtualization-host-xen.html

I don't have plans to backport for 13.0, but will consider backporting
to stable/13 at a later point if there are no issues, so the
functionality could be used in 13.1.

This has been tested on a single Intel box with UEFI, some wider
testing would be appreciated.

Thanks to tsoome, imp and kib for the reviews.

Roger.