tools/libxc: Remove osdep indirection for xc_gnt{shr,tab}

tools/libxc: Remove osdep indirection for xc_gnt{shr,tab}

The alternative backend (a xen-api/xapi shim) is no longer around and
so this stuff is now just baggage which is getting in the way of
refactoring libxenctrl.

Nested virt probably suffices for this use case now.

It is now necessary to provide explicit versions of things for
platforms which do not implement this functionality, since the osdep
dispatcher cannot fulfil this need any more. These are provided by
appropriate xc_nognt???.c files which are compiled and linked on the
appropriate platforms. In them open and close return failure and
everything else aborts, since if open fails they should never be
called.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>