[XENSTORED] Fix errno 'leak' in xenstored.

[XENSTORED] Fix errno 'leak' in xenstored.

In certain cases, when a client doesn't have enough permissions, the
errno variable is not set in xenstored_core.c before its value is
reported back.  As a result, the client can learn about the errno of
the last failed request to xenstored (which could have come from
another client).  (An unintended information channel! :-)

From: Magnus Carlsson <magnus@galois.com>
Signed-off-by: Keir Fraser <keir@xensource.com>