Update Xen headers from 4.2 to 4.6

Pull the latest headers for Xen which allow us to add support for ARM and
use new features in FreeBSD.

This is a verbatim copy of the xen/include/public so every headers which
don't exits anymore in the Xen repositories have been dropped.

Note the interface version hasn't been bumped, it will be done in a
follow-up. Although, it requires fix in the code to get it compiled:

• sys/xen/xen_intr.h: evtchn_port_t is already defined in the headers so drop it.
• {amd64,i386}/include/intr_machdep.h: NR_EVENT_CHANNELS now depends on xen/interface/event_channel.h, so include it.
• {amd64,i386}/{amd64,i386}/support.S: It's not neccessary to include machine/intr_machdep.h. This is also fixing build compilation with the new headers.
• dev/xen/blkfront/blkfront.c: The typedef for blkif_request_segmenthas been dropped. So directly use struct blkif_request_segment

Finally, modify xen/interface/xen-compat.h to throw a preprocessing error if
XEN_INTERFACE_VERSION is not set. This is allow us to catch any file
where xen/xen-os.h is not correctly included.

Submitted by: Julien Grall <julien.grall@citrix.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D3805
Sponsored by: Citrix Systems R&D