tools: Do not expose all builds to unstable hypercall APIs by default

tools: Do not expose all builds to unstable hypercall APIs by default

Currently __XEN_TOOLS__ is defined in the global CFLAGS used for tools
builds, exposing them all to the various unstable hypercall interfaces.

However there are places where we do not want this (in particular all
the stable libraries now under tools/libs), and there is a risk of
such uses slipping in unnoticed (currently there are none, but I
tripped over the mixed nature of the HVMOPs just now in some new code
I was putting together).

Instead only define __XEN_TOOLS__ for things which explicitly want the
unstable APIs. The vast majority of these are consumers of libxenctrl,
which have opted into unstable interfaces by using libxenctrl already,
so adding -D__XEN_TOOLS__ to CFLAGS_libxenctrl covers these without
needing to patch the dozen or so tools manually.

The libxc build itself, which doesn't use CFLAGS_libxenctrl, now needs
the define too. As does gdbsx's own low level hypercall binding
library xg.

hvmloader no longer needs to undefine the symbol.

The ocaml evtchn bindings don't actually need sysctl.h and therefore
don't actually need __XEN_TOOLS__ either.

qemu-xen-traditional doesn't use CFLAGS_libxenctrl and so we pass the
define there (via --extra-cflags) too.

Having removed __XEN_TOOLS__ replace it with
__XEN_INTERFACE_VERSION__=__XEN_LATEST_INTERFACE_VERSION__ so that in
tree code is always using the latest APIs.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>