libxc: provide notification of final checkpoint to restore end

libxc: provide notification of final checkpoint to restore end

When the restore code sees this notification it will restore the
currently in-progress checkpoint when it completes.

This allows the restore end to finish up without waiting for a
spurious timeout on the receive fd and thereby avoids unnecessary
error logging in the case of a successful migration or restore.

In the normal migration or restore case the first checkpoint is always
the last. For a rolling checkpoint (such as Remus) the notification is
currently unused but could be used in the future for example to
provide a controlled failover for reasons other than error

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Brendan Cully <brendan@cs.ubc.ca>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>