tools/xenstore: Re-introduce (fake) xs_restrict call to preserve ABI

tools/xenstore: Re-introduce (fake) xs_restrict call to preserve ABI

libxenstore3.0 in Xen 4.8 had this function.  We don't really want to
bump the ABI version (soname) just for this, since we don't think
there are actual callers anywhere.  But tools complain about the
symbol going away.

So, provide a function xs_restrict which conforms to the original
semantics, although it always fails.

Gbp-Pq: Topic xenstore
Gbp-Pq: Name tools-fake-xs-restrict.patch
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>