xen: import Xen 4.16 public headers in sys/contrib/

The current path of the Xen headers at /sys/xen/interface/ is not
correct, as those headers are imported verbatim from the Xen sources
and shouldn't be modified, as any modifications would be lost when a
new version is imported.

Changes to the public headers must be first done in Xen upstream so
that they can be backported and new imports will already carry them.

Import Xen 4.16 headers in sys/contrib/xen/. It's unlikely that we
will import different Xen code, so don't place them inside of any
subdirectory. If in the future other pieces of Xen code need to be
imported the headers will need to move into an include/ subdirectory.

Note that this commit does not yet modify the include path to use the
newly imported headers.

Sponsored by: Citrix Systems R&D