xen/dmop: Strip __XEN_TOOLS__ header guard from public ABI

xen/dmop: Strip __XEN_TOOLS__ header guard from public ABI

__XEN_TOOLS__ is really there to separate the unstable from stable hypercalls.
Exactly as with c/s f40e1c52e4, stable interfaces shouldn't contain this
guard.

That change actually broke the build with:

  include/xendevicemodel.h:52:5: error: unknown type name 'ioservid_t'
       ioservid_t *id);
       ^

as libxendevicemodel.h now uses a type it can't see a typedef for.  However,
nothing noticed because the header.chk logic is also broken (fixed
subsequently).

Strip the guard from the public header, and remove compensation from
devicemodel's private.h.  Fix the dmop design doc to discuss both reasons
behind the the ABI design.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>