xenstore_write doesn't exist, use xenstore-write instead. The error
function is currently broken without this change.
Signed-off-by: Roger Pau Monne <roger.pau@citrix.com> Signed-off-by: Christoph Egger <Christoph.Egger@amd.com> Acked-by: Ian Jackson <ian.jackson@eu.citrix.com> Committed-by: Ian Campbell <ian.campbell@citrix.com>