tools/xenstored: Make gnttab interface mandatory

tools/xenstored: Make gnttab interface mandatory

xenstored currently requires an libxc and evtchn interface, but leaves
the gnttab interface as optional.

gnttab is ubiquitous these days, and in practice mandatory in all cases
where xenstored isn't running as root in dom0 (due to the inability to
foreign map by MFN).

The toolstack has unconditionally set up grant details for many years
now, and longterm it would be good to phase out the use of libxc.  This
requires that xenstored map the store ring by grant map, rather than
foreign map.

No practical change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>