Default credentials

When a transaction is committed, C Xenstored will first check
the quota is correct before attempting to commit any nodes. It would
be possible that accounting is temporarily negative if a node has
been removed outside of the transaction.

Unfortunately, some versions of C Xenstored are assuming that the
quota cannot be negative and are using assert() to confirm it. This
will lead to C Xenstored crash when tools are built without -DNDEBUG
(this is the default).