Fix handling of the entries-per-domain quota. Entries which are created by

Fix handling of the entries-per-domain quota.  Entries which are created by
the guest but deleted by dom0 were remaining accounted against the guest,
which meant that the guest would eventually run out of quota.

This patch also prevents unprivileged domains from changing the owner of a
node.  One guest could attack another by creating nodes and then transferring
them to the ownership of another, and though the accounting could be made to
work properly in this case, domains should never be transferring nodes in any
case, so it seems safer just to disallow the operation entirely.

Signed-off-by: Ewan Mellor <ewan@xensource.com>