tools/xenstored: Drop mapping of the ring via foreign map

tools/xenstored: Drop mapping of the ring via foreign map

This is a vestigial remnent of the pre xenstored stub domain days.

Foreign mapping via MFN is a privileged operation which is not
necessary, because grant details are unconditionally set up during
domain construction.  In practice, this means xenstored never uses its
ability to foreign map the ring.

Drop the ability completely, which removes the penultimate use of the
unstable libxc interface.

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