xenbus: allow any xenbus command over /proc/xen/xenbus

When xenstored is in another domain, we need to be able to send any
command over xenbus.  This doesn't pose a security problem because
its up to xenstored to determine whether a given client is allowed
to use a particular command anyway.

From linux-2.5.18-xen.hg 68d582b0ad05.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>