xenstore: remove XS_RESTRICT support

xenstore: remove XS_RESTRICT support

XS_RESTRICT and the xenstore library function xs_restrict() have never
been usable in all configurations and there are no known users.

This functionality was thought to limit access rights of device models
to xenstore in order to avoid affecting other domains in case of a
security breech. Unfortunately XS_RESTRICT won't help as current
qemu is requiring access to dom0 only accessible xenstore paths to
work correctly. So this command is useless and should be removed.

In order to avoid problems in the future remove all support for
XS_RESTRICT from xenstore.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: David Scott <dave@recoil.org>