include: import Xen public headers to include/standard-headers/

include: import Xen public headers to include/standard-headers/

There are already some partial headers in include/hw/xen/interface/
which will be removed once we migrate users to the new location.

To start with, define __XEN_TOOLS__ in hw/xen/xen.h to ensure that any
internal definitions needed by Xen toolstack libraries are present
regardless of the order in which the headers are included. A reckoning
will come later, once we make the PV backends work in emulation and
untangle the headers for Xen-native vs. generic parts.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
[dwmw2: Update to Xen public headers from 4.16.2 release, add some in io/,
        define __XEN_TOOLS__ in hw/xen/xen.h]
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>