When a domain is introduced to xenstored, check whether this has already

When a domain is introduced to xenstored, check whether this has already
happened, and only return EINVAL if the event channel details have changed.
This allows Xend to introduce domains when it starts without having to be
concerned about whether xenstored has restarted at the same time.  This
behaviour used to be subsumed by the bind_interdomain semantics for existing
channels, but in the simplification of that interface, the check must now move
to userspace.

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