xen: fetch dom0 video console information from Xen

It's possible for Xen to switch the video mode set by the boot loader,
so that the information passed in the kernel metadata is no longer
valid. Fetch the video mode used by Xen using an hypercall and update
the medatada for the kernel to use the correct video mode.

Sponsored by: Citrix Systems R&D

xen/acpi: upload Cx and Px data to Xen

When FreeBSD is running as dom0 (initial domain) on a Xen system it
has access to the native ACPI tables and is the OSPM. However the
hypervisor is the entity in charge of the CPU idle and frequency
states, and in order to perform this duty it requires information
found the ACPI dynamic tables that can only be parsed by the OSPM.

Introduce a new Xen specific ACPI driver to fetch the Processor
related information and upload it to Xen. Note that this driver needs
to take precedence over the generic ACPI CPU driver when running as
dom0, so downgrade the probe score of the native driver to
BUS_PROBE_DEFAULT in order for the Xen specific driver to use
BUS_PROBE_SPECIFIC.

Tested on an Intel NUC to successfully parse and upload both the Cx and
Px states to Xen.

Sponsored by: Citrix Systems R&D
Reviewed by: jhb kib
Differential revision: https://reviews.freebsd.org/D34841

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