xl: do not return to caller from monitoring daemon

xl: do not return to caller from monitoring daemon

The parent process will have returned to the caller and done whatever
is necessary. The daemon should not return otherwise it will repeat
this work. In the case of the migration receiver this causes it to try
and take part in the migration protocol long after the sender+parent
process have completed it, leading to confusing error messages
(although strangely not much actual damange).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>