xenstore-utils on debian wheezy does not have /usr/sbin/xenstore so these commands file. It does have xenstore-write and xenstore-rm so by adding a - this is fixed easily.

Signed-off-by: Daan Hoogland <daan@onecht.net>
(cherry picked from commit 2e83baaca2c998cfdda281301dffb09a1a20aa5c)